address_types.c: fix a "conditional jump or move depends on uninitialised value" error found by Valgrind

Change-Id: Ic752bb6aa1ba56e113895fcbed52f417ac0128f7
Reviewed-on: https://code.wireshark.org/review/7272
Reviewed-by: Pascal Quantin <pascal.quantin@gmail.com>

epan/address_types.c

@@ -492,7 +492,7 @@ static int fcwwn_to_str(const address* addr, gchar *buf, int buf_len _U_)
     const guint8 *addrp = (const guint8*)addr->data;
 
     buf = bytes_to_hexstr_punct(buf, addrp, 8, ':'); /* 23 bytes */
-    buf[23] = '\0';
+    *buf = '\0';
 
     return fcwwn_str_len(addr);
 }