st/mesa: fix glCopyTexSubImage crash

st/mesa: fix glCopyTexSubImage crash

Fixes a WebGL crash.  The dest texture image is at level 2 and is of
size 1x1 texel.  The st texture image is a stand-alone resource, not
a pointer into a complete mipmap.  So the resource has one level and
trying to write to level 2 blows up.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=53314
and http://bugs.freedesktop.org/show_bug.cgi?id=53319

Note: This is a candidate for the 8.0 branch.

Reviewed-by: Jose Fonseca <jfonseca@vmware.com>