dns (tcp) - fix coverity CIDs 1374306, 1374305

CID 1374306 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking dns_state suggests that it may be null,
but it has already been dereferenced on all paths leading to the check.
585    if (dns_state != NULL && f != NULL) {
586        dns_state->last_req = f->lastts;
587    }

CID 1374305 (#1 of 1): Dereference before null check (REVERSE_INULL)
check_after_deref: Null-checking dns_state suggests that it may be null,
but it has already been dereferenced on all paths leading to the check.
366    if (dns_state != NULL && f != NULL) {
367        dns_state->last_req = f->lastts;
368    }

src/app-layer-dns-tcp.c

@@ -363,7 +363,7 @@ static int DNSTCPRequestParse(Flow *f, void *dstate,
             goto bad_data;
     }
 
-    if (dns_state != NULL && f != NULL) {
+    if (f != NULL) {
         dns_state->last_req = f->lastts;
     }
 
@@ -582,7 +582,7 @@ static int DNSTCPResponseParse(Flow *f, void *dstate,
             goto bad_data;
     }
 
-    if (dns_state != NULL && f != NULL) {
+    if (f != NULL) {
         dns_state->last_req = f->lastts;
     }