Fix gamma test not checking for NULL return value

(cherry picked from commit 8aaea57421)

tests/gamma.c

@@ -111,6 +111,12 @@ int main(int argc, char** argv)
 
     {
         const GLFWgammaramp* ramp = glfwGetGammaRamp(monitor);
+        if (!ramp)
+        {
+            glfwTerminate();
+            exit(EXIT_FAILURE);
+        }
+
         const size_t array_size = ramp->size * sizeof(short);
         orig_ramp.size = ramp->size;
         orig_ramp.red = malloc(array_size);