fw_env: Add missing write failure check

If flash_write fails, whilst we propagate this up to our caller, we need
to avoid swapping in the new file (if we're on a filesystem) in this
case.

Fixes: dbc3432379 ("tools: env: Implement atomic replace for filesystem")
Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>

tools/env/fw_env.c

@@ -1317,7 +1317,7 @@ static int flash_io_write(int fd_current)
 			rc = -1;
 		}
 
-		if (target_temp) {
+		if (rc >= 0 && target_temp) {
 			int dir_fd;
 
 			dir_fd = open(dname, O_DIRECTORY | O_RDONLY);