glx: fix glGetTexLevelParameteriv for indirect rendering

glx: fix glGetTexLevelParameteriv for indirect rendering

A single element in a GLX reply is contained in the header itself.
The number of elements is denoted in the "n" field of the reply.
If "n" is 1, the length of additional data is 0.
The XXX_data_length() function of xcb does not return the length of
the (optional, n>1) data but the number of elements.

Fixes http://bugs.freedesktop.org/show_bug.cgi?id=59876

Note: This is a candidate for the stable branches.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>