mesa: fix proxy textures not working with default texture binding

mesa: fix proxy textures not working with default texture binding

When working with the glTexStorage*() functions, the error checking
checks that a non-default (i.e., non-zero) texture is currently bound.
However, this check made glTexStorage*() functions fail with proxy
textures when the default texture is bound. Proxy textures do not care
about the current texture bindings so for them this check should not
be done.

Reviewed-by: Brian Paul <brianp@vmware.com>
Cc: mesa-stable@lists.freedesktop.org