Stack reallocation done in two phases

$he stack reallocation is done in two steps (allocation + free) because
the correction of the pointers pointing into the stack must be done
while both addresses (the old stack and the new one) are valid.  In ISO
C, any pointer use after the pointer has been deallocated is undefined
behavior. The compiler option '-fsanitize=pointer-subtract' (plus what
it needs to work) complained about the old implementation.

ldo.c

@@ -160,8 +160,6 @@ int luaD_rawrunprotected (lua_State *L, Pfunc f, void *ud) {
 static void correctstack (lua_State *L, StkId oldstack, StkId newstack) {
   CallInfo *ci;
   UpVal *up;
-  if (oldstack == newstack)
-    return;  /* stack address did not change */
   L->top = (L->top - oldstack) + newstack;
   L->tbclist = (L->tbclist - oldstack) + newstack;
   for (up = L->openupval; up != NULL; up = up->u.open.next)
@@ -179,19 +177,35 @@ static void correctstack (lua_State *L, StkId oldstack, StkId newstack) {
 #define ERRORSTACKSIZE	(LUAI_MAXSTACK + 200)
 
 
+/*
+** Reallocate the stack to a new size, correcting all pointers into
+** it. (There are pointers to a stack from its upvalues, from its list
+** of call infos, plus a few individual pointers.) The reallocation is
+** done in two steps (allocation + free) because the correction must be
+** done while both addresses (the old stack and the new one) are valid.
+** (In ISO C, any pointer use after the pointer has been deallocated is
+** undefined behavior.)
+** In case of allocation error, raise an error or return false according
+** to 'raiseerror'.
+*/
 int luaD_reallocstack (lua_State *L, int newsize, int raiseerror) {
-  int lim = stacksize(L);
-  StkId newstack = luaM_reallocvector(L, L->stack,
-                      lim + EXTRA_STACK, newsize + EXTRA_STACK, StackValue);
+  int oldsize = stacksize(L);
+  int i;
+  StkId newstack = luaM_reallocvector(L, NULL, 0,
+                                      newsize + EXTRA_STACK, StackValue);
   lua_assert(newsize <= LUAI_MAXSTACK || newsize == ERRORSTACKSIZE);
   if (l_unlikely(newstack == NULL)) {  /* reallocation failed? */
     if (raiseerror)
       luaM_error(L);
     else return 0;  /* do not raise an error */
   }
-  for (; lim < newsize; lim++)
-    setnilvalue(s2v(newstack + lim + EXTRA_STACK)); /* erase new segment */
+  /* number of elements to be copied to the new stack */
+  i = ((oldsize <= newsize) ? oldsize : newsize) + EXTRA_STACK;
+  memcpy(newstack, L->stack, i * sizeof(StackValue));
+  for (; i < newsize + EXTRA_STACK; i++)
+    setnilvalue(s2v(newstack + i)); /* erase new segment */
   correctstack(L, L->stack, newstack);
+  luaM_freearray(L, L->stack, oldsize + EXTRA_STACK);
   L->stack = newstack;
   L->stack_last = L->stack + newsize;
   return 1;