Can accidentially overwrite .smd files

[TruePikachu]

A mistyped invocation for pmDJ can result in accidentially overwriting a .smd file with .mid data

• pmDJ foo.smd bar.smd - Overwrites bar.smd with MIDI data from foo.smd
• pmDJ foo.smd bar.smd baz.mid - (intended command) Writes MIDI data to baz.mid

Solution would be to check the file extension on the argv[argc-1] argument.

[TruePikachu]

Quick note I'm not putting in the issue report proper, eventually, we might have pmDJ foo.mid bar.smd, so it might be beneficial to check all the file extensions and validate them.