suricatasc: Handle incomplete/empty recv values

Issue: 4947

Improve handling of values returned by recv. Sometimes, recv returns an
empty string if suricata terminates asynchronously.

(cherry picked from commit fc6fdef070)

python/suricata/sc/suricatasc.py

@@ -119,9 +119,14 @@ class SuricataSC:
         data = ""
         while True:
             if sys.version < '3':
-                data += self.socket.recv(INC_SIZE)
+                received = self.socket.recv(INC_SIZE)
             else:
-                data += self.socket.recv(INC_SIZE).decode('iso-8859-1')
+                received = self.socket.recv(INC_SIZE).decode('iso-8859-1')
+
+            if not received:
+                break
+
+            data += received
             if data.endswith('\n'):
                 cmdret = json.loads(data)
                 break