gdb_main: Fix remaining length calculation for 'X' 'M' and 'vFlashWrite'

When removing use of sscanf from gdb_main the calculation of the length
of the remaining data portion of the 'X', 'M', and 'vFlashWrite' packet
types was done wrongly. The difference between the start of data pointer
and start of buffer pointer was reversed resulting in adding this to the
total length rather than subtracting it.  For the 'X' and 'M' packets
this causes them to accept lengths longer than the supplied data but
correct packets would still function as expected. For the 'vFlashWrite'
packet the length passed to the flash write function was about 8 larger
than it should have been which could cause problems.

The calculation of the data length portion of the packets has been
corrected so all three now work as expected.

src/gdb_main.c

@@ -168,7 +168,7 @@ int32_t gdb_main_loop(target_controller_s *tc, char *pbuf, size_t pbuf_size, siz
 		uint32_t len = 0;
 		ERROR_IF_NO_TARGET();
 		if (read_hex32(pbuf + 1, &rest, &addr, ',') && read_hex32(rest, &rest, &len, ':')) {
-			if (len > (size - (size_t)(pbuf - rest)) / 2U) {
+			if (len > (size - (size_t)(rest - pbuf)) / 2U) {
 				gdb_putpacketz("E02");
 				break;
 			}
@@ -344,7 +344,7 @@ int32_t gdb_main_loop(target_controller_s *tc, char *pbuf, size_t pbuf_size, siz
 		uint32_t addr, len;
 		ERROR_IF_NO_TARGET();
 		if (read_hex32(pbuf + 1, &rest, &addr, ',') && read_hex32(rest, &rest, &len, ':')) {
-			if (len > (size - (size_t)(pbuf - rest))) {
+			if (len > (size - (size_t)(rest - pbuf))) {
 				gdb_putpacketz("E02");
 				break;
 			}
@@ -789,7 +789,7 @@ static void exec_v_flash_write(const char *packet, const size_t length)
 	const char *rest = NULL;
 	if (read_hex32(packet, &rest, &addr, ':')) {
 		/* Write Flash Memory */
-		const uint32_t count = length - (packet - rest);
+		const uint32_t count = length - (size_t)(rest - packet);
 		DEBUG_GDB("Flash Write %08" PRIX32 " %08" PRIX32 "\n", addr, count);
 		if (cur_target && target_flash_write(cur_target, addr, (uint8_t *)rest, count))
 			gdb_putpacketz("OK");