locking: check two magic bytes, to verify that it is a lock file

Also, when the check fails, then nano should continue and simply
open the file, just like Vim.

This fixes https://savannah.gnu.org/bugs/?57698.

src/files.c

@@ -333,13 +333,20 @@ int do_lockfile(const char *filename, bool ask_the_user)
 
 		close(lockfd);
 
-		if (readtot < 48) {
+		if (readtot < 1024) {
 			statusline(MILD, _("Error reading lock file %s: "
 						"Not enough data read"), lockfilename);
 			free(lockbuf);
 			goto free_the_name;
 		}
 
+		if (lockbuf[0] != 0x62 || lockbuf[1] != 0x30) {
+			statusline(ALERT, _("Bad lock file is ignored: %s"), lockfilename);
+			free(lockbuf);
+			retval = 0;
+			goto free_the_name;
+		}
+
 		strncpy(lockprog, &lockbuf[2], 10);
 		lockpid = (((unsigned char)lockbuf[27] * 256 + (unsigned char)lockbuf[26]) * 256 +
 						(unsigned char)lockbuf[25]) * 256 + (unsigned char)lockbuf[24];