Fix DoS vulnerability in DHO_OPTIONSOVERLOADED.

Bug: 16677003

(cherry picked from commit 371e319488e62475a4185a93bf47580860268561)

Change-Id: I01446092c7ca9e0133d835cfdae8a9ee467d7aaf
diff --git a/dhcp.c b/dhcp.c
index c22e767..53f4795 100644
--- a/dhcp.c
+++ b/dhcp.c
@@ -352,7 +352,7 @@
 		case DHO_OPTIONSOVERLOADED:
 			/* Ensure we only get this option once */
 			if (!overl)
-				overl = p[1];
+				overl = 0x80 | p[1];
 			break;
 		}
 		l = *p++;