Fix double fclose() in an error case.

CID:		1006120
Found with:	Coverity Prevent(tm)
MFC after:	1 week
mergemaster: Avoid "/var/tmp/temproot disappeared" if there is nothing to
compare.

Because of the change to find in SVN r253886, the entire temproot would be
deleted if it became empty, leading to a confusing message "*** FATAL ERROR:
The temproot directory ${TEMPROOT} has disappeared!"

Note that mergemaster does not do anything useful in this situation anyway
(e.g. put IGNORE_FILES="/etc/group /etc/master.passwd" in
/etc/mergemaster.rc and run mergemaster -p).

As noted in that commit, add -mindepth 1.

PR:		bin/188485
Submitted by:	David Boyd
MFC after:	1 week