mesa: Handle negative length in glPushDebugGroup().

mesa: Handle negative length in glPushDebugGroup().

The KHR_debug spec doesn't actually say we should handle this, but that
is most likely an oversight - it says to check against strlen and
generate errors if length is negative.  It appears they just forgot to
explicitly spell out that we should then proceed to actually handle it.

Fixes crashes from uncaught std::string exceptions in many
dEQP-GLES31.functional.debug.error_filters.* tests.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eduardo Lima Mitev <elima@igalia.com>