mesa: In validate_program(), initialize errMsg for safety.

mesa: In validate_program(), initialize errMsg for safety.

validate_program relies on validate_shader_program to fill in errMsg;
empirically, there exist cases where that doesn't happen.

While tracking those down may be worthwhile, initializing the string so
we don't try to ralloc_strdup random garbage also seems wise.

Fixes issues caught by valgrind while running some test case.

NOTE: This is a candidate for stable release branches.

Reviewed-by: Chad Versace <chad@chad-versace.us>
Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>