Avoid gcc warning on FreeBSD.

Avoid gcc warning on FreeBSD.

FreeBSD 9.1 ships a patched gcc-4.2.1 which complains about use of
an initialized variable:

	fd.c: In function 'for_each_file_in_dir':
	fd.c:728: warning: 'dir' may be used uninitialized in this function

This warning is bogus since para_opendir() only leaves the dir
pointer unset on failures. But in this case for_each_file_in_dir()
returns early without using the pointer.

This commit changes para_opendir() to set dir to NULL before doing
anything else. This avoids the warning at almost no cost.