Use ssize_t instead of short for line lengths

Use ssize_t instead of short for line lengths

sthen hit a binary patch containing a 'line' of length > 32kB. This made
the short used for storing the line length wrap and resulted in a buffer
underflow and segfault.  This uses a larger type, which doesn't actually
fix the problem, but makes it much less likely to be hit.

ok florian otto sthen