Various bugfixes

While working on the mingw64 port, I found and squashed the following little bugs in our current codebase:

• pd/extra/fiddle~/fiddle~.c: sigfiddle_doinit(): removed bogus ! operator (backported from vanilla rev. 2c98b929)
• pd/src/import.c: import_output(): fix potentially unterminated strncpy
• externals/bassemu~/bassemu~.c: bassemu_list(): bogus assignment, changed to equality

Note that this MR is based on !514 (merged), so you can kill two birds with one stone by just merging this one and ignoring the other. :)