jtag/hla_interface: avoid segfault with adapters that do not have configurable speed

Change-Id: I0386cbfc85ba8b28d3819530f9950b31545d6821
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/2468
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>

src/jtag/hla/hla_interface.c

@@ -150,6 +150,9 @@ int hl_interface_init_reset(void)
 
 static int hl_interface_khz(int khz, int *jtag_speed)
 {
+	if (hl_if.layout->api->speed == NULL)
+		return ERROR_OK;
+
 	*jtag_speed = hl_if.layout->api->speed(hl_if.handle, khz, true);
 	return ERROR_OK;
 }