Issue 550: Fix out-of-bounds read in mtree.

Issue 550:  Fix out-of-bounds read in mtree.

The mtree parser scanned from the end of the string to identify
the filename when the filename is the last element of the line.
If the filename was the entire line, the logic would scan back
to before the start of the string.

The revised logic scans from the beginning of the string
and remembers the last separator position to locate the
trailing filename.