Make the format parameter to string_format an obect, not a reference

Apparently va_start does not like to work with references.
Fix one Mac compilation bug

See #6770.

This doesn't change the actual function of the code, just silences an error from the finicky mac compiler.
Merge branch 'fix-format_string-function' of https://github.com/BevapDin/Cataclysm-DDA into BevapDin-fix-format_string-function
Merge branch 'patch-7' of https://github.com/Izicata/Cataclysm-DDA into Izicata-patch-7