selftests/net: initialize char variable to null

char variable in 'so_txtime.c' & 'txtimestamp.c' were left uninitilized
when switch default case taken. which raises following warning.

txtimestamp.c:240:2: warning: variable 'tsname' is used uninitialized
whenever switch default is taken [-Wsometimes-uninitialized]

so_txtime.c:210:3: warning: variable 'reason' is used uninitialized
whenever switch default is taken [-Wsometimes-uninitialized]

initializing these variables to NULL to fix this.

Signed-off-by: Ankit Khushwaha <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/so_txtime.c

···
 	msg.msg_controllen = sizeof(control);
 
 	while (1) {
-		const char *reason;
+		const char *reason = NULL;
 
 		ret = recvmsg(fdt, &msg, MSG_ERRQUEUE);
 		if (ret == -1 && errno == EAGAIN)

tools/testing/selftests/net/txtimestamp.c

···
 static void print_timestamp(struct scm_timestamping *tss, int tstype,
 			    int tskey, int payload_len)
 {
-	const char *tsname;
+	const char *tsname = NULL;
 
 	validate_key(tskey, tstype);