Fix mbsnrtowcs where `dst` is null.

POSIX is its usual unintelligible self
(http://pubs.opengroup.org/onlinepubs/9699919799/functions/mbsrtowcs.html),
but the ISO C11 standard (7.29.6.4.1 paragraph 2) is pretty clear: *src
should change if and only if dst is non-null.

Bug: https://code.google.com/p/android/issues/detail?id=166381
Test: bionic tests
Change-Id: Ibc631cfa5b1bf4a6f56963feba9f0eea27b07984