apply: keep buffer/size pair in sync when parsing binary hunks

We parse through binary hunks by looping through the buffer with code like: llen = linelen(buffer, size); ...do something with the line... buffer += llen; size -= llen; However, before we enter the loop, there is one call that increments "buffer" but forgets to decrement "size". As a result, our "size" is off by the length of that line, and subsequent calls to linelen() may look past the end of the buffer for a newline. The fix is easy: we just need to decrement size as we do elsewhere. This bug goes all the way back to 0660626caf (binary diff: further updates., 2006-05-05). Presumably nobody noticed because it only triggers if the patch is corrupted, and even then we are often "saved" by luck. We use a strbuf to store the incoming patch, so we overallocate there, plus we add a 16-byte run of NULs as slop for memory comparisons. So if this happened accidentally, the common case is that we'd just read a few uninitialized bytes from the end of the strbuf before producing the expected "this patch is corrupted" error complaint. However, it is possible to carefully construct a case which reads off the end of the buffer. The included test does so. It will pass both before and after this patch when run normally, but using a tool like ASan shows that we get an out-of-bounds read before this patch, but not after. Reported-by: Xingman Chen <xichixingman@gmail.com> Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>