mntent: fix potential mishandling of extremely long lines

commit 05973dc3bb made it so that lines
longer than INT_MAX can in theory be read, but did not use a suitable
type for the positions determined by sscanf. we could change to using
size_t, but since the signature for getmntent_r does not admit lines
longer than INT_MAX, it does not make sense to support them in the
legacy thread-unsafe form either -- the principle here is that there
should not be an incentive to use the unsafe function to get added
functionality.

src/misc/mntent.c

@@ -2,6 +2,7 @@
 #include <string.h>
 #include <mntent.h>
 #include <errno.h>
+#include <limits.h>
 
 static char *internal_buf;
 static size_t internal_bufsize;
@@ -42,6 +43,7 @@ struct mntent *getmntent_r(FILE *f, struct mntent *mnt, char *linebuf, int bufle
 		}
 
 		len = strlen(linebuf);
+		if (len > INT_MAX) continue;
 		for (i = 0; i < sizeof n / sizeof *n; i++) n[i] = len;
 		sscanf(linebuf, " %n%*s%n %n%*s%n %n%*s%n %n%*s%n %d %d",
 			n, n+1, n+2, n+3, n+4, n+5, n+6, n+7,