libblkid: ignore extended partition at zero offset

If the extended partition starts at zero LBA then MBR is interpreted
as EBR and all is recursively parsed... result is out-of-memory.

 MBR --extended-partition--> EBR --> MBR --> ENB --> MBR ...

Note that such PT is not possible to create by standard partitioning
tools.

Addresses: https://bugzilla.redhat.com/show_bug.cgi?id=1349536
Signed-off-by: Karel Zak <kzak@redhat.com>

libblkid/src/partitions/dos.c

@@ -47,6 +47,12 @@ static int parse_dos_extended(blkid_probe pr, blkid_parttable tab,
 	int ct_nodata = 0;	/* count ext.partitions without data partitions */
 	int i;
 
+	DBG(LOWPROBE, ul_debug("parse EBR [start=%d, size=%d]", ex_start/ssf, ex_size/ssf));
+	if (ex_start == 0) {
+		DBG(LOWPROBE, ul_debug("Bad offset in primary extended partition -- ignore"));
+		return 0;
+	}
+
 	while (1) {
 		struct dos_partition *p, *p0;
 		uint32_t start, size;
@@ -116,8 +122,12 @@ static int parse_dos_extended(blkid_probe pr, blkid_parttable tab,
 			start = dos_partition_get_start(p) * ssf;
 			size = dos_partition_get_size(p) * ssf;
 
-			if (size && is_extended(p))
-				break;
+			if (size && is_extended(p)) {
+				if (start == 0)
+					DBG(LOWPROBE, ul_debug("#%d: EBR link offset is zero -- ignore", i + 1));
+				else
+					break;
+			}
 		}
 		if (i == 4)
 			goto leave;