gl: return NULL mem if eglCreateImage failed

Found on rpi when gpu_mem is too low so there is not enough memory to
create the eglimage. But still gst_buffer_pool_acquire_buffer succeeded.
And it leads to a CRITICAL assert:
  gst_egl_image_get_image: assertion 'GST_IS_EGL_IMAGE (image)' failed

https://bugzilla.gnome.org/show_bug.cgi?id=785518

gst-libs/gst/gl/egl/gstglmemoryegl.c

@@ -155,6 +155,11 @@ _gl_mem_egl_alloc (GstGLBaseMemoryAllocator * allocator,
       params->parent.alloc_params, params->v_info, params->plane,
       params->valign, params->parent.user_data, params->parent.notify);
 
+  if (!mem->image) {
+    gst_allocator_free (GST_ALLOCATOR_CAST (allocator), GST_MEMORY_CAST (mem));
+    return NULL;
+  }
+
   return mem;
 }