mesa/objectlabel: handle NULL src string

This prevents a crash when a NULL src is passed with a non-NULL length.

fixes: dEQP-GLES31.functional.debug.object_labels.query_length_only
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=95252

Signed-off-by: Mark Janes <mark.a.janes@intel.com>
Reviewed-by: Dave Airlie <airlied@redhat.com>

src/mesa/main/objectlabel.c

@@ -104,14 +104,15 @@ copy_label(const GLchar *src, GLchar *dst, GLsizei *length, GLsizei bufSize)
     * will be returned in <length>."
     */
 
-   if (bufSize == 0) {
-      if (length)
-         *length = strlen(src);
-      return;
-   }
    if (src)
       labelLen = strlen(src);
 
+   if (bufSize == 0) {
+      if (length)
+         *length = labelLen;
+      return;
+   }
+
    if (dst) {
       if (src) {
          if (bufSize <= labelLen)