gl_fence_manager: Minor optimization to signal querying

Per the spec, bufSize is the number of integers that will be written, in this case, 1.

Also, the length argument is optional if the information of the number of elements written is not needed.

src/video_core/renderer_opengl/gl_fence_manager.cpp

@@ -31,9 +31,8 @@ bool GLInnerFence::IsSignaled() const {
         return true;
     }
     ASSERT(sync_object.handle != 0);
-    GLsizei length;
     GLint sync_status;
-    glGetSynciv(sync_object.handle, GL_SYNC_STATUS, sizeof(GLint), &length, &sync_status);
+    glGetSynciv(sync_object.handle, GL_SYNC_STATUS, 1, nullptr, &sync_status);
     return sync_status == GL_SIGNALED;
 }