Also don't treat an empty buffer as JSON.

That also keeps us from looking at the non-existent first octet of an
empty buffer.

Bug: 16031
Change-Id: I3fcf4201d21dc44ccd8815cb0637c1eae4995560
Reviewed-on: https://code.wireshark.org/review/34439
Petri-Dish: Guy Harris <[email protected]>
Tested-by: Petri Dish Buildbot
Reviewed-by: Guy Harris <[email protected]>

wsutil/wsjson.c

@@ -37,12 +37,16 @@ json_validate(const guint8 *buf, const size_t len)
         return FALSE;
 
     /*
-     * Make sure the first octet isn't a NUL; otherwise, the parser will
-     * immediately stop parsing and not validate anything after that,
-     * so it'll just think it was handed an empty string.
+     * Make sure the buffer isn't empty and the first octet isn't a NUL;
+     * otherwise, the parser will immediately stop parsing and not validate
+     * anything after that, so it'll just think it was handed an empty string.
      *
      * XXX - should we check for NULs anywhere in the buffer?
      */
+    if (len == 0) {
+        g_log(LOG_DOMAIN_MAIN, G_LOG_LEVEL_DEBUG, "jsmn: JSON string is empty");
+        return FALSE;
+    }
     if (buf[0] == '\0') {
         g_log(LOG_DOMAIN_MAIN, G_LOG_LEVEL_DEBUG, "jsmn: invalid character inside JSON string");
         return FALSE;