Avoid out-of-range PC for stack overflow error from snapshot restore.

Reported by Sergey Kaplun.

(cherry picked from commit e3fa3c4)

In case when the saved PC in the snapshot is the first (0th index) PC in
the prototype like JFUNC*, the subtraction to determine the previous PC
in the `debug_framepc()` overflows and contains `NO_BCPOS` value. After
that, the pos is greater than sizebc. Hence, the code below may
interpret the bits in `pt->varinfo` like `bc_isret()` and assign an
invalid value to `pos` to be returned. Further, it may lead to the
assertion failure in the lj_debug_frameline().

This patch fixes it by pretending that this means the first non-header
bytecode in the prototype. Also, this patch removes the skipcond
introduced in the commit a74e5be
("test: conditionally disable flaky lj-1196"). The new test isn't added
since the assertion failure depends on the specific memory address of
the `varinfo`, so it is too hard to create a stable reproducer.

Sergey Kaplun:
* added the description for the problem

Part of tarantool/tarantool#11691

Reviewed-by: Sergey Bronnikov <[email protected]>
Signed-off-by: Sergey Kaplun <[email protected]>

Author: Mike Pall

src/lj_debug.c

@@ -101,6 +101,7 @@ static BCPos debug_framepc(lua_State *L, GCfunc *fn, cTValue *nextframe)
   pt = funcproto(fn);
   pos = proto_bcpos(pt, ins) - 1;
 #if LJ_HASJIT
+  if (pos == NO_BCPOS) return 1;  /* Pretend it's the first bytecode. */
   if (pos > pt->sizebc) {  /* Undo the effects of lj_trace_exit for JLOOP. */
     if (bc_isret(bc_op(ins[-1]))) {
       GCtrace *T = (GCtrace *)((char *)(ins-1) - offsetof(GCtrace, startins));

test/tarantool-tests/lj-1196-partial-snap-restore.test.lua

@@ -4,15 +4,7 @@ local tap = require('tap')
 -- in case of the stack overflow.
 -- See also: https://github.com/LuaJIT/LuaJIT/issues/1196.
 
-local test = tap.test('lj-1196-partial-snap-restore'):skipcond({
-  -- Disable test for Tarantool to avoid failures, see also:
-  -- https://github.com/LuaJIT/LuaJIT/issues/1369.
-  ['Disabled for Tarantool due to lj-1369'] = _TARANTOOL,
-  -- Also, it may fail on some non-arm64 runners stable after
-  -- adding the skip condition above.
-  ['Disabled for x86/x64 due to lj-1369'] = jit.arch ~= 'arm64',
-})
-
+local test = tap.test('lj-1196-partial-snap-restore')
 test:plan(1)
 
 -- XXX: The reproducer below uses several stack slot offsets to