spi/flashconsole: Fix internal buffer overflow

Once the console's FMAP region is full, we stop clearing the line
buffer and `line_offset` is not reset anymore. Hence, sanity check
`line_offset` everytime before writing to the buffer.

The issue resulted in boot hangs and potentially a brick if the
log was very verbose.

Change-Id: I36e9037d7baf8c1ed8b2d0c120bfffa58c089c95
Signed-off-by: Nico Huber <nico.huber@secunet.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/48074
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Michael Niewöhner <foss@mniewoehner.de>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/drivers/spi/flashconsole.c

@@ -75,7 +75,8 @@ void flashconsole_tx_byte(unsigned char c)
 
 	size_t region_size = region_device_sz(rdev_ptr);
 
-	line_buffer[line_offset++] = c;
+	if (line_offset < LINE_BUFFER_SIZE)
+		line_buffer[line_offset++] = c;
 
 	if (line_offset >= LINE_BUFFER_SIZE ||
 	    offset + line_offset >= region_size || c == '\n') {