avoid expression with side effect (in debug mode) inside 'sizeof'

ldump.c

@@ -1,5 +1,5 @@
 /*
-** $Id: ldump.c,v 2.34 2014/11/02 19:19:04 roberto Exp roberto $
+** $Id: ldump.c,v 2.35 2015/01/16 16:54:37 roberto Exp roberto $
 ** save precompiled Lua chunks
 ** See Copyright Notice in lua.h
 */
@@ -75,13 +75,14 @@ static void DumpString (const TString *s, DumpState *D) {
     DumpByte(0, D);
   else {
     size_t size = tsslen(s) + 1;  /* include trailing '\0' */
+    const char *str = getstr(s);
     if (size < 0xFF)
       DumpByte(cast_int(size), D);
     else {
       DumpByte(0xFF, D);
       DumpVar(size, D);
     }
-    DumpVector(getstr(s), size - 1, D);  /* no need to save '\0' */
+    DumpVector(str, size - 1, D);  /* no need to save '\0' */
   }
 }