PRVM_GarbageCollection: Add range check on string garbage collection (#294)

When using FTEQCC structs, it's possible for string variables to be
outside the string global space. This can cause the garbage collector to
segfault. This PR checks that the index is within bounds and will
display a debug message and skip any that are not.

Author: hemebond

prvm_edict.c

@@ -3817,6 +3817,12 @@ void PRVM_GarbageCollection(prvm_prog_t *prog)
 					if (s & PRVM_KNOWNSTRINGBASE)
 					{
 						prvm_int_t num = s - PRVM_KNOWNSTRINGBASE;
+
+						if (num < 0 || num >= prog->numknownstrings)
+						{
+							continue;
+						}
+
 						if (!prog->knownstrings[num])
 						{
 							// invalid