EVM: prevent array underflow in evm_write_xattrs()

If the user sets xattr->name[0] to NUL then we would read one character
before the start of the array.  This bug seems harmless as far as I can
see but perhaps it would trigger a warning in KASAN.

Fixes: fa516b66 ("EVM: Allow runtime modification of the set of verified xattrs")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mimi Zohar <zohar@linux.vnet.ibm.com>