gdb_server: fix segfault with GDB command 'flash-erase'

Running the GDB command 'flash-erase' triggers sending the remote
GDB commands 'vFlashErase' (one per flash bank) followed by one
single 'vFlashDone', with no 'vFlashWrite' commands in between.
This causes the field 'gdb_connection->vflash_image' to be NULL
during the execution of 'vFlashDone', triggering a segmentation
fault in OpenOCD.

While parsing 'vFlashDone', check if any image to flash has been
received.

Change-Id: I443021c7a531255b60f2c44c2685e52e3c34b5c8
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/8164
Tested-by: jenkins

src/server/gdb_server.c

@@ -3376,6 +3376,13 @@ static int gdb_v_packet(struct connection *connection,
 	if (strncmp(packet, "vFlashDone", 10) == 0) {
 		uint32_t written;
 
+		/* GDB command 'flash-erase' does not send a vFlashWrite,
+		 * so nothing to write here. */
+		if (!gdb_connection->vflash_image) {
+			gdb_put_packet(connection, "OK", 2);
+			return ERROR_OK;
+		}
+
 		/* process the flashing buffer. No need to erase as GDB
 		 * always issues a vFlashErase first. */
 		target_call_event_callbacks(target,