mesa: eliminate debug output message_insert

mesa: eliminate debug output message_insert

Add validate_length, and call it together with log_msg directly instead of
message_insert.  No functional change.

v2: make sure length is non-negative (i.e., known) before calling
    validate_length, noted by Timothy Arceri

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Brian Paul <brianp@vmware.com>