validate: tools: transcoding error due to wrong condition check

when checking the restriction caps, not adding proper check, which
results in assertion error when calling gst_caps_from_string

https://bugzilla.gnome.org/show_bug.cgi?id=752749

validate/tools/gst-validate-transcoding.c

@@ -598,7 +598,7 @@ _parse_encoding_profile (const gchar * option_name, const gchar * value,
     encoding_profile = NULL;
   }
 
-  for (i = 1; strcaps_v[i]; i++) {
+  for (i = 1; strcaps_v[i] && *strcaps_v[i]; i++) {
     GstEncodingProfile *profile = NULL;
     gchar *strcaps, *strpresence;