Add pa_sample_format_valid()

I think this makes the code a bit nicer to read and write. This also
reduces the chances of off-by-one errors when checking the bounds of
the sample format value.

src/map-file

@@ -251,6 +251,7 @@ pa_rtclock_now;
 pa_sample_format_is_be;
 pa_sample_format_is_le;
 pa_sample_format_to_string;
+pa_sample_format_valid;
 pa_sample_size;
 pa_sample_size_of_format;
 pa_sample_spec_equal;