rtspconnection: Fix potential deadlock caused by blocking read forever

Reset the connection "may_cancel" property to avoid a potential deadlock
if there is no data to read and the socket stays blocked forever.

https://bugzilla.gnome.org/show_bug.cgi?id=768249

gst-libs/gst/rtsp/gstrtspconnection.c

@@ -2161,11 +2161,13 @@ build_next (GstRTSPBuilder * builder, GstRTSPMessage * message,
     }
   }
 done:
+  conn->may_cancel = TRUE;
   return res;
 
   /* ERRORS */
 invalid_body_len:
   {
+    conn->may_cancel = TRUE;
     GST_DEBUG ("could not allocate body");
     return GST_RTSP_ERROR;
   }