[TEXTSEARCH]: Do not allow zero length patterns in the textsearch infrastructure

If a zero length pattern is passed then return EINVAL.
Avoids infinite loops (bm) or invalid memory accesses (kmp).

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>