encoding.c: avoid UBSAN warning in convert_string()

encoding.c: avoid UBSAN warning in convert_string()

When 'in == NULL && in_nbytes == 0', the statement 'in_end = in +
in_nbytes' executes 'NULL + 0'.  clang's UndefinedBehaviorSanitizer
complains that this is undefined:

    src/encoding.c:223:31: runtime error: applying zero offset to null pointer

This is questionable, but let's avoid it...