jtag/core: fix segfault when adapter driver has no reset method

xlnx-pcie-xvc and linuxspidev adapter drivers does not implement
the reset method.
Although it is likely both adapters will implement the method in
the near future, avoid segfault and return an error instead.

Change-Id: If8ddf165dbc563cf6d64b2094968151075778ba7
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Fixes: commit 8850eb8 ("swd: get rid of jtag queue to assert/deassert srst")
Reviewed-on: https://review.openocd.org/c/openocd/+/8735
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

Author: tom-van

src/jtag/core.c

@@ -631,6 +631,13 @@ static int adapter_system_reset(int req_srst)
 
 	/* Maybe change SRST signal state */
 	if (jtag_srst != req_srst) {
+		if (!adapter_driver->reset) {
+			if (req_srst)
+				LOG_ERROR("Adapter driver does not implement SRST handling");
+
+			return ERROR_NOT_IMPLEMENTED;
+		}
+
 		retval = adapter_driver->reset(0, req_srst);
 		if (retval != ERROR_OK) {
 			LOG_ERROR("SRST error");