Improve parsing of certain FTP directory listing formats (#2408)

Improve parsing of certain FTP directory listing formats (#2408)

This surgical fix restricts parsing to the input buffer when the listing
entry date in "TypeA" or "TypeB" formats is not followed by a filename.
It does not improve rendering of listings with missing filenames or the
overall quality of FTP listing parsing code.

C strchr() always returns a non-nil pointer when given a NUL character,
so its callers must be careful not to supply a NUL character if a
"natural" one-of-the-regular-c-string-characters membership test is
required.

The bug was probably introduced in 1997 commit 3fdadc70 and then
duplicated in 2017 commit 3d872090.