Some details in 'lmem.c' and 'lgc.c'

- Several new comments in 'lmem.c'.
- Both 'luaM_growaux_' and 'luaM_shrinkvector_' use 'luaM_saferealloc_'
  to check for errors. Moreover, the use of 'luaM_saferealloc_' makes
  'luaM_shrinkvector_' try again if shrink fails (which can happen now).
- In 'checkSizes', save old debt only when needed.

lgc.c

@@ -794,10 +794,11 @@ static GCObject **sweeptolive (lua_State *L, GCObject **p) {
 */
 static void checkSizes (lua_State *L, global_State *g) {
   if (!g->gcemergency) {
-    l_mem olddebt = g->GCdebt;
-    if (g->strt.nuse < g->strt.size / 4)  /* string table too big? */
+    if (g->strt.nuse < g->strt.size / 4) {  /* string table too big? */
+      l_mem olddebt = g->GCdebt;
       luaS_resize(L, g->strt.size / 2);
-    g->GCestimate += g->GCdebt - olddebt;  /* correct estimate */
+      g->GCestimate += g->GCdebt - olddebt;  /* correct estimate */
+    }
   }
 }