mesa: modified _mesa_align_free() to accept NULL pointer

mesa: modified _mesa_align_free() to accept NULL pointer

So that it acts like ordinary free().  This lets us remove a bunch of
if statements where the function is called.

v2:
- Avoiding compile error on MSVC and possible warnings on other compilers.
- Added comment regards passing NULL pointer being safe.

Reviewed-by: Brian Paul <brianp@vmware.com>