Bug 1290328 - Return null for webgl GetActiveUniformBlockName() error. r=jgilbert

dom/canvas/WebGL2ContextUniforms.cpp

@@ -277,6 +277,7 @@ void
 WebGL2Context::GetActiveUniformBlockName(WebGLProgram* program, GLuint uniformBlockIndex,
                                          nsAString& retval)
 {
+    retval.SetIsVoid(true);
     if (IsContextLost())
         return;