MFC r336115;
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)