GitLab

Two issues are tackled:

1. The calculation of the correct length to print is fixed to treat the
   precision as a maximum length instead as the exact length.
   This is done for both qstr (%q) and for regular str (%s).

2. Fix the incorrect use of mp_printf("%.*s") to mp_print_strn().

   Because of the fix of above issue, some testcases that would print
   an embedded null-byte (^@ in test-output) would now fail.
   The bug here is that "%s" was used to print null-bytes. Instead,
   mp_print_strn is used to make sure all bytes are outputted and the
   exact length is respected.

Test-cases are added for both %s and %q with a combination of precision
and padding specifiers.