swrast: fix assorted bugs in software blit code

swrast: fix assorted bugs in software blit code

1. The loop over dest buffers in blit_linear() needed a null pointer
check.  Fixes https://bugs.freedesktop.org/show_bug.cgi?id=59499

2. The code to grab the drawRb's format needs to be inside the drawing loop.

3. An equality test was using = instead of == thus messing up a
renderbuffer attachment texture pointer.  This lead to memory
corruption and a crash at exit.

Finally, fix a capitalization error NumDrawBuffers -> numDrawBuffers
and change type to unsigned to fix signed/unsigned comparison warnings.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>