Bug 1751238 - Handle empty device extension strings in glxtest, r=gfx-reviewers,jgilbert

While the string should never be empty when the extension is actually
supported (which is checked above), there seem to be edge-cases where
that is the case.
Handle that gracefully.

Differential Revision: https://phabricator.services.mozilla.com/D136534

toolkit/xre/glxtest.cpp

@@ -541,7 +541,8 @@ static bool get_gles_status(EGLDisplay dpy,
       EGL_TRUE) {
     const char* deviceExtensions =
         eglQueryDeviceStringEXT(device, EGL_EXTENSIONS);
-    if (strstr(deviceExtensions, "EGL_MESA_device_software")) {
+    if (deviceExtensions &&
+        strstr(deviceExtensions, "EGL_MESA_device_software")) {
       record_value("MESA_ACCELERATED\nFALSE\n");
     } else {
 #ifdef MOZ_WAYLAND