mesa/swrast: Move free calls outside the attachment loop

mesa/swrast: Move free calls outside the attachment loop

This was originally discovered by Klocwork analysis:

    Possible memory leak. Dynamic memory stored in 'srcBuffer0'
    allocated through function 'malloc' at line 566 can be lost at line
    746

However, I think the problem is actually much worse.  Since the memory
is freed after the first pass through the loop, the released buffer may
be used on the next iteration!

NOTE: This is a candidate for stable release branches.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>