isis: add some additional length checks.

Make sure the length in question is big enough before fetching data and
subtracting from it.
diff --git a/print-isoclns.c b/print-isoclns.c
index b5943cc..3859c8f 100644
--- a/print-isoclns.c
+++ b/print-isoclns.c
@@ -1921,10 +1921,14 @@
 	    algo = GET_U_1(tptr+1);
 
 	    if (flags & ISIS_PREFIX_SID_FLAG_V) {
+	        if (subl < 5)
+	            goto trunc;
 		sid = GET_BE_U_3(tptr+2);
 		tptr+=5;
 		subl-=5;
 	    } else {
+	        if (subl < 6)
+	            goto trunc;
 		sid = GET_BE_U_4(tptr+2);
 		tptr+=6;
 		subl-=6;