Return 0, not -1, for "this isn't my type of file".

-1 means "I got an error reading this file, so there's no point in
trying any more open routines".  It doesn't mean "I couldn't find any
matching pattern in the text"; that's 0, for "this isn't my type of
file, but keep trying".

Change-Id: I9d2e8b8fe6720052cacf70f0bacdcbc1175202cc
Reviewed-on: https://code.wireshark.org/review/3674
Reviewed-by: Guy Harris <guy@alum.mit.edu>