bt: fix use of uninitialized variable seqlen

sdp_svc_match, sdp_attr_match and sdp_svc_attr_match read the last
argument.  The only sensible way to change the code is to make that last
argument "len" instead of "seqlen" which is the length of a subsequence
in the previous "if" branch.

To make the structure of the code clearer, use "else" instead of
"else if".

Reported by Coverity.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
