[NETFILTER]: nf_conntrack_amanda: fix textsearch_prepare() error check

The return value from textsearch_prepare() needs to be checked
by IS_ERR(). Because it returns error code as a pointer.

Cc: "Brian J. Murrell" <netfilter@interlinx.bc.ca>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>