GLX: Fix segfault when no GLXFBConfigs available

Fixes #1040.

README.md

@@ -208,6 +208,8 @@ information on what to include when reporting a bug.
 - [WGL] Added support for `WGL_EXT_colorspace` for OpenGL ES contexts
 - [WGL] Added support for `WGL_ARB_create_context_no_error`
 - [GLX] Added support for `GLX_ARB_create_context_no_error`
+- [GLX] Bugfix: Context creation could segfault if no GLXFBConfigs were
+                available (#1040)
 - [EGL] Added support for `EGL_KHR_get_all_proc_addresses` (#871)
 - [EGL] Added support for `EGL_KHR_context_flush_control`
 - [EGL] Bugfix: The test for `EGL_RGB_BUFFER` was invalid

src/glx_context.c

@@ -64,7 +64,7 @@ static GLFWbool chooseGLXFBConfig(const _GLFWfbconfig* desired, GLXFBConfig* res
 
     nativeConfigs =
         glXGetFBConfigs(_glfw.x11.display, _glfw.x11.screen, &nativeCount);
-    if (!nativeCount)
+    if (!nativeConfigs || !nativeCount)
     {
         _glfwInputError(GLFW_API_UNAVAILABLE, "GLX: No GLXFBConfigs returned");
         return GLFW_FALSE;