test: Avoid egl_without_glx crash on NVIDIA

The NVIDIA driver calls dlopen with NULL as filename; strcmp is not
NULL-safe.

test/egl_without_glx.c

@@ -54,6 +54,7 @@ dlopen(const char *filename, int flag)
 {
     void * (*dlopen_unwrapped)(const char *filename, int flag);
 
+    if (filename) {
         if (!strcmp(filename, "libGL.so.1"))
             return NULL;
 #if GLES_VERSION == 2
@@ -63,6 +64,7 @@ dlopen(const char *filename, int flag)
         if (!strcmp(filename, "libGLESv2.so.2"))
             return NULL;
 #endif
+    }
 
     dlopen_unwrapped = dlsym(RTLD_NEXT, "dlopen");
     assert(dlopen_unwrapped);