wtap_file_read_till_separator() can return values other than 1 on success.

It returns the length of the string it read, so only treat 0 and -1 as
errors.  (0 either means "EOF" or "string is zero length", but this is
only in the code that reads numbers, and a number needs at least 1
digit, so both EOF and "zero-length string" mean "this isn't a valid
Peek tagged file".)

Change-Id: Ib83eb2f1e53d912a2138be01480e2b464cf936db
Reviewed-on: https://code.wireshark.org/review/4591
Reviewed-by: Guy Harris <guy@alum.mit.edu>