mesa: avoid segfault in GetProgramPipelineInfoLog when no length
mesa: avoid segfault in GetProgramPipelineInfoLog when no length

If there is no pipe info log, we would unconditionally deref length,
which was only optionally there. _mesa_copy_string handles the source
being null, as well as the length, so may as well just always call it.

Fixes a segfault in

dEQP-GLES31.functional.state_query.program_pipeline.info_log

Signed-off-by: Ilia Mirkin <bcbbb7aa705ec3a5fc5824c01a845186c3d62fca@alum.mit.edu>
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler