bufferpool: Fix the buffer size reset code

bufferpool: Fix the buffer size reset code

The offset in gst_buffer_resize() is additive. So to move back the
offset to zero, we need to pass the opposite of the current offset. This
was raised through the related unit test failingon 32bit as on 64bit
the alignment padding was enough to hide the issue. The test was
modified to also fail on 64bit. This patch will remove spurious
assertions like:

  assertion 'bufmax >= bufoffs + offset + size' failed

Fixes #316