funct(): Fix another use after free bug (re: f24040e, 69d37d5) (#519)

The ASan crash in basic.sh when sourcing multiple files is caused by
a bug that is similar to the crash fixed in f24040e. This is the
trace for the regression test crash (note that in order to see the
trace, the 2>/dev/null redirect must be disabled):

==1899388==ERROR: AddressSanitizer: heap-use-after-free on address 0x6150000005b0 at pc 0x55a5e3f9432a bp 0x7ffeb91ea110 sp 0x7ffeb91ea100
WRITE of size 8 at 0x6150000005b0 thread T0
    #0 0x55a5e3f94329 in funct /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:967
    ksh-community#1 0x55a5e3f96f77 in item /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:1349
    ksh-community#2 0x55a5e3f90c9f in term /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:642
    ksh-community#3 0x55a5e3f90ac1 in list /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:613
    ksh-community#4 0x55a5e3f90845 in sh_cmd /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:561
    ksh-community#5 0x55a5e3f909e0 in sh_cmd /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:586
    ksh-community#6 0x55a5e3f8fd5e in sh_parse /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/parse.c:438
    ksh-community#7 0x55a5e3fc43c1 in sh_eval /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/xec.c:635
    ksh-community#8 0x55a5e4012172 in b_dot_cmd /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/misc.c:318
    ksh-community#9 0x55a5e3fca3cb in sh_exec /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/xec.c:1254
    ksh-community#10 0x55a5e3fd01d4 in sh_exec /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/xec.c:1932
    ksh-community#11 0x55a5e3fc4544 in sh_eval /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/xec.c:651
    ksh-community#12 0x55a5e4012172 in b_dot_cmd /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/bltins/misc.c:318
    ksh-community#13 0x55a5e3fca3cb in sh_exec /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/xec.c:1254
    ksh-community#14 0x55a5e3ecc1cd in exfile /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/main.c:604
    ksh-community#15 0x55a5e3ec9e7f in sh_main /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/main.c:369
    ksh-community#16 0x55a5e3ec801d in main /home/johno/GitRepos/KornShell/ksh/src/cmd/ksh93/sh/pmain.c:41
    ksh-community#17 0x7f637b4db2cf  (/usr/lib/libc.so.6+0x232cf)
    ksh-community#18 0x7f637b4db389 in __libc_start_main (/usr/lib/libc.so.6+0x23389)
    ksh-community#19 0x55a5e3ec7f24 in _start ../sysdeps/x86_64/start.S:115

Code in question:
https://github.com/ksh93/ksh/blob/8d57369b0cb39074437dd82924b604155e30e1e0/src/cmd/ksh93/sh/parse.c#L963-L968

To avoid any more similar crashes, all of the fixes introduced
in 69d37d5 that set slp->slptr to null have been improved with the
fix in f24040e.

Author: JohnoKing

src/cmd/ksh93/sh/name.c

@@ -2466,8 +2466,9 @@ void	_nv_unset(register Namval_t *np,int flags)
 			}
 			if(slp->slptr)
 			{
-				stakdelete(slp->slptr);
+				Stak_t *sp = slp->slptr;
 				slp->slptr = NIL(Stak_t*);
+				stakdelete(sp);
 			}
 			free((void*)np->nvalue.ip);
 			np->nvalue.ip = 0;

src/cmd/ksh93/sh/parse.c

@@ -963,8 +963,9 @@ static Shnode_t *funct(Lex_t *lexp)
 		if(slp && slp->slptr)
 		{
 			sh.st.staklist = slp->slnext;
-			stakdelete(slp->slptr);
+			Stak_t *slptr_save = slp->slptr;
 			slp->slptr = NIL(Stak_t*);
+			stakdelete(slptr_save);
 		}
 		siglongjmp(*sh.jmplist,jmpval);
 	}

src/cmd/ksh93/sh/xec.c

@@ -1411,8 +1411,9 @@ int sh_exec(register const Shnode_t *t, int flags)
 					sh_funstaks(slp->slchild,-1);
 					if(slp->slptr)
 					{
-						stakdelete(slp->slptr);
+						Stak_t *sp = slp->slptr;
 						slp->slptr = NIL(Stak_t*);
+						stakdelete(sp);
 					}
 					if(jmpval > SH_JMPFUN || (io && jmpval > SH_JMPIO))
 						siglongjmp(*sh.jmplist,jmpval);
@@ -2462,8 +2463,9 @@ int sh_exec(register const Shnode_t *t, int flags)
 				sh_funstaks(slp->slchild,-1);
 				if(slp->slptr)
 				{
-					stakdelete(slp->slptr);
+					Stak_t *sp = slp->slptr;
 					slp->slptr = NIL(Stak_t*);
+					stakdelete(sp);
 				}
 				if(rp->sdict)
 				{