ethercat: add default case in FoeFormatter

Should probably fix an unintialized memory access caught by valgrind, although I
can't reproduce it because out-of-tree plugins are still broken.

Bug: 10919
Change-Id: Ib8c46e13922f25260ca0e8886368f5ce24e3d0b0
Reviewed-on: https://code.wireshark.org/review/6962
Reviewed-by: Guy Harris <guy@alum.mit.edu>
Reviewed-by: Anders Broman <a.broman58@gmail.com>

plugins/ethercat/packet-ecatmb.c

@@ -405,6 +405,8 @@ static void FoeFormatter(tvbuff_t *tvb, gint offset, char *szText, gint nMax, gu
       else
          g_snprintf ( szText, nMax, "FoE BUSY (%d/%d)", foe.aFoeHeaderDataUnion.v2.Done, foe.aFoeHeaderDataUnion.v2.Entire);
       break;
+   default:
+      g_snprintf ( szText, nMax, "FoE Unknown");
    }
 }