libiconv: correct undefined behavior.

Detected on NetBSD:

nm /usr/lib/libc.so|grep sanit

/public/src.git/lib/libc/citrus/modules/citrus_mapper_std.c:173:8:

runtime error: left shift of 1 by 31 places cannot be represented in type 'int'

Obtained from: NetBSD (CVS Rev. 1.11)
MFC after: 1 week