base64: use stricter validation on wrap column

base64: use stricter validation on wrap column

* src/base64.c (main): Use the higher level xnumtoumax()
rather than xstrtoumax(), which is simpler and improves
validation of input.  Also pass the _empty_ rather than NULL
string as the suffixes parameter so that invalid trailing
characters are not allowed.  For example -w08 is now
flagged as an error, rather than being interpreted as 0.
A subsequent commit will further improve verification
of numbers with leading zeros by dropping backwards compatibility
wrt auto parsing oct and hex numbers.
* tests/misc/base64.pl: Add tests for invalid wrap values.