mesa: handle missing read buffer in _mesa_get_color_read_format/type()

mesa: handle missing read buffer in _mesa_get_color_read_format/type()

We were crashing when GL_READ_BUFFER == GL_NONE.  Check for NULL
pointers and reorganize the code.  The spec doesn't say which error
to generate in this situation, but NVIDIA raises GL_INVALID_OPERATION.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=65173
NOTE: This is a candidate for the stable branches.

Tested-by: Vedran Rodic <vrodic@gmail.com>
Reviewed-by: José Fonseca <jfonseca@vmware.com>