glsl_to_tgsi: fix a reference leak on an error path

src/mesa/state_tracker/st_glsl_to_tgsi.cpp

@@ -5050,6 +5050,7 @@ st_link_shader(struct gl_context *ctx, struct gl_shader_program *prog)
             break;
          }
          if (!ok) {
+            _mesa_reference_program(ctx, &linked_prog, NULL);
             return GL_FALSE;
          }
       }