lib/cgi.c (cgi__input): Write terminating null within allocated buffer.

lib/cgi.c (cgi__input): Write terminating null within allocated buffer.

This one's been there forever.  It sometimes results in `malloc'
reporting a (rather impressive) assertion failure

	disorder: malloc.c:3096: sYSMALLOc: Assertion `(old_top ==
	(((mbinptr) (((char *) &((av)->bins[((1) - 1) * 2])) -
	__builtin_offsetof (struct malloc_chunk, fd)))) &&
	old_size == 0) || ((unsigned long) (old_size) >=
	(unsigned long)((((__builtin_offsetof (struct malloc_chunk,
	fd_nextsize))+((2 * (sizeof(size_t))) - 1)) & ~((2 *
	(sizeof(size_t))) - 1))) && ((old_top)->size & 0x1) &&
	((unsigned long)old_end & pagemask) == 0)' failed.

often after producing the beginning of the output page, so the end user
gets a blank window.  Not really what we want.

This fix closes issue #57.