libexport.a: export_find() should handle address parsing errors

libexport.a: export_find() should handle address parsing errors

An address mask parsing error can cause client_init(), and therefore
client_dup(), to make our process exit suddenly.  Soon we want to add
more complex address parsing in client_init(), so we need this
interface to be a little more robust.

Since export_find() can return NULL in some cases, it can handle NULL
returns from its subroutines if an address parsing error occurs, or if
memory is exhausted.  Allow for client_dup() to return NULL instead of
exiting sideways.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Steve Dickson <steved@redhat.com>