mesa: added null ptr checks

src/mesa/shader/prog_uniform.c

 _mesa_lookup_uniform(const struct gl_uniform_list *list, const char *name)
 {
    GLuint i;
-   for (i = 0; i < list->NumUniforms; i++) {
+   for (i = 0; list && i < list->NumUniforms; i++) {
       if (!_mesa_strcmp(list->Uniforms[i].Name, name)) {
          return i;
       }
 {
    GLint max = 0;
    GLuint i;
-   for (i = 0; i < list->NumUniforms; i++) {
+   for (i = 0; list && i < list->NumUniforms; i++) {
       GLuint len = _mesa_strlen(list->Uniforms[i].Name);
       if (len > (GLuint)max)
          max = len;