Fix underlying bugs. mrb_calloc will be crashed in case "nelem == 0" or "p == NULL"

src/gc.c

@@ -167,10 +167,15 @@ mrb_malloc(mrb_state *mrb, size_t len)
 void*
 mrb_calloc(mrb_state *mrb, size_t nelem, size_t len)
 {
-  void *p = mrb_realloc(mrb, 0, nelem*len);
+  void *p;
+  size_t size;
+
+  size = nelem * len;
+  p = mrb_realloc(mrb, 0, size);
+
+  if (p && size > 0)
+    memset(p, 0, size);
 
-  if (len > 0)
-    memset(p, 0, nelem*len);
   return p;
 }