mesa: Add condition in glGetTexImage for zero size textures

TestMipMaps() function in src/OGLconform/textureNPOT.c calls glTexImage2D()
with width = 0. Texture with zero size skips miptree allocation due to a
condition in function _mesa_store_teximage3d(). While calling glGetTexImage()
it results in assertion failure in intel_map_texture_image() due to null mt
pointer.

This patch fixes the issue by detecting the zero size texture early in
glGetTexImage and glGetCompressedTexImage functions. In such a case function
simply returns doing nothing.
Verified that below mentioned bug is fixed by this patch.

https://bugs.freedesktop.org/show_bug.cgi?id=42334

NOTE: This is a candidate for stable branches

Signed-off-by: Anuj Phogat <[email protected]>
Reviewed-by: Brian Paul <[email protected]>

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