Fix #189, ensure returns an invalid pointer

If realloc returns NULL, ensure didn't abort but returned
printbuffer.offset instead. If an attacker can control
printbuffer.offset and also make realloc fail at just the right moment,
this would make cJSON potentially write at an arbitrary memory address.

cJSON.c

@@ -377,6 +377,14 @@ static unsigned char* ensure(printbuffer * const p, size_t needed)
     {
         /* reallocate with realloc if available */
         newbuffer = (unsigned char*)p->hooks.reallocate(p->buffer, newsize);
+        if (newbuffer == NULL)
+        {
+            p->hooks.deallocate(p->buffer);
+            p->length = 0;
+            p->buffer = NULL;
+
+            return NULL;
+        }
     }
     else
     {