Minor cleanups

Remove useless checks: coverity 1038130, 1038131, 1038132
Small other fixes: coverity 1164817

src/app-layer-dcerpc-udp.c

@@ -796,10 +796,7 @@ static void DCERPCUDPStateFree(void *s) {
         sstate->dcerpc.dcerpcresponse.stub_data_buffer = NULL;
         sstate->dcerpc.dcerpcresponse.stub_data_buffer_len = 0;
     }
-	if (s) {
-		SCFree(s);
-		s = NULL;
-	}
+    SCFree(s);
 }
 
 static int DCERPCUDPRegisterPatternsForProtocolDetection(void)

src/app-layer-dcerpc.c

@@ -1976,10 +1976,7 @@ static void DCERPCStateFree(void *s) {
         sstate->dcerpc.dcerpcresponse.stub_data_buffer_len = 0;
     }
 
-    if (s) {
-        SCFree(s);
-        s = NULL;
-    }
+    SCFree(s);
 }
 
 static int DCERPCRegisterPatternsForProtocolDetection(void)

src/app-layer-dns-common.c

@@ -76,7 +76,7 @@ void DNSDecrMemcap(uint32_t size, DNSState *state) {
     }
 
     BUG_ON(size > SC_ATOMIC_GET(dns_memuse)); /**< TODO remove later */
-    SC_ATOMIC_SUB(dns_memuse, size);
+    (void)SC_ATOMIC_SUB(dns_memuse, size);
 }
 
 int DNSCheckMemcap(uint32_t want, DNSState *state) {

src/app-layer-smb.c

@@ -1455,11 +1455,7 @@ static void SMBStateFree(void *s) {
         sstate->dcerpc.dcerpcresponse.stub_data_buffer_len = 0;
     }
 
-    if (s) {
-        SCFree(s);
-        s = NULL;
-    }
-
+    SCFree(s);
     SCReturn;
 }