iconvctl(3): remove superfluous NULL pointer tests

convname and dst are guaranteed to be non-NULL by iconv_open(3).
src is an array. Remove these tests for NULL pointers.
While I'm here, eliminate a strlcpy with a correct but suspicious-looking
calculation for the third parameter (i.e. not a simple sizeof).
Compare the strings in-place instead of copying.

Found by: bdrewery
Found by: Coverity
CID: 1130050, 1130056
MFC after: 3 days
Sponsored by: Dell Inc.
Differential Revision: https://reviews.freebsd.org/D6338