Generate GL_INVALID_OPERATION for texSubImage2D if no previously defined texture image
Generate GL_INVALID_OPERATION for texSubImage2D if no previously defined texture image

This fixes conformance test tex-sub-image-2d-bad-args.html.

BUG=295792

Review URL: https://codereview.chromium.org/1280613002

git-svn-id: bf5cd6ccde378db821296732a091cfbcf5285fbd@200189 bbb929c8-8fbe-4397-9dbb-9b2b20218538

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