dns: suppress harmless cppcheck warning

[src/app-layer-dns-common.c:273]: (warning) Assignment of function \
                        parameter has no effect outside the function.
dns: suppress minor scan-build warnings

These were only used if debug is enabled.

app-layer-dns-tcp.c:407:13: warning: Value stored to 'length' is never read
            length = *data;
app-layer-dns-udp.c:236:13: warning: Value stored to 'length' is never read
            length = *data;
stream: suppress minor scan-build warnings

stream-tcp-reassemble.c:2569:17: warning: Value stored to 'seg' is never read
                seg = seg->next;
                ^     ~~~~~~~~~
stream-tcp-reassemble.c:2587:17: warning: Value stored to 'seg' is never read
                seg = seg->next;
detect-ssl: suppress harmless scan-build warning

detect-ssl-version.c:271:17: warning: Value stored to 'neg' is never read
                neg = 0;