Disallow filenames containing NUL bytes.

C code gets confused by such filenames, and even if we can handle them,
they're a misleading and of questionable utility because a valid pathname
can't contain a NUL byte.

Bug: https://code.google.com/p/android/issues/detail?id=58465
Bug: 10148349
Change-Id: I3f7e1a17c526b6bb5752dae82610125ad2648d54
(cherry picked from commit b1d05573ae20cdd3d5b4e25f905ce44c8f7d5c0e)