pack-bitmap: bounds-check size of cache extension

A .bitmap file may have a "name hash cache" extension, which puts a sequence of uint32_t values (one per object) at the end of the file. When we see a flag indicating this extension, we blindly subtract the appropriate number of bytes from our available length. However, if the .bitmap file is too short, we'll underflow our length variable and wrap around, thinking we have a very large length. This can lead to reading out-of-bounds bytes while loading individual ewah bitmaps. We can fix this by checking the number of available bytes when we parse the header. The existing "truncated bitmap" test is now split into two tests: one where we don't have this extension at all (and hence actually do try to read a truncated ewah bitmap) and one where we realize up-front that we can't even fit in the cache structure. We'll check stderr in each case to make sure we hit the error we're expecting. Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Taylor Blau <me@ttaylorr.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>