ftdi: Fix error in calloc retval check

Found by clang.

Change-Id: Iaa952b063779f4d392d8bdc62a3e7ce9daef2740
Signed-off-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>
Reviewed-on: http://openocd.zylin.com/1498
Tested-by: jenkins
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/drivers/ftdi.c

@@ -127,8 +127,10 @@ static struct signal *create_signal(const char *name)
 		psig = &(*psig)->next;
 
 	*psig = calloc(1, sizeof(**psig));
-	if (*psig)
-		(*psig)->name = strdup(name);
+	if (*psig == NULL)
+		return NULL;
+
+	(*psig)->name = strdup(name);
 	if ((*psig)->name == NULL) {
 		free(*psig);
 		*psig = NULL;