check for sdp_content in sanity

kamailio/sanity.cfg

@@ -65,10 +65,12 @@ route[SANITY_CHECK]
         drop;
     }
 
-    if(sdp_get_line_startswith("$avp(sanity_sline)", "s=")) {
-        if ($avp(sanity_sline) == "s=portsip.com") {
-            xlog("L_WARN", "$ci|end|dropping message with '$avp(sanity_sline)' string in SDP\n");
-            exit;
+    if(sdp_content()) {
+        if(sdp_get_line_startswith("$avp(sanity_sline)", "s=")) {
+            if ($avp(sanity_sline) == "s=portsip.com") {
+                xlog("L_WARN", "$ci|end|dropping message with '$avp(sanity_sline)' string in SDP\n");
+                exit;
+            }
         }
     }
 }