#13 (1.6.0) Wrong register format string is accepted

ArchC 1.6.0

A wrong register format string such as

ac_format Fmt_RRR = "%field2:16 field1:16";

is accepted. Note that "field1" should have a "%"
character before it. Quite interestingly, register RRR
does have "field1" correctly declared in the generated
code. Maybe "%" is intentionally optional?