Fix an integer overflow in RLE length parsing when decompressing

corrupt bzip2 data.

Approved by:	so (cperciva)
Security:	FreeBSD-SA-10:08.bzip2

contrib/bzip2/decompress.c

@@ -381,6 +381,13 @@ Int32 BZ2_decompress ( DState* s )
             es = -1;
             N = 1;
             do {
+               /* Check that N doesn't get too big, so that es doesn't
+                  go negative.  The maximum value that can be
+                  RUNA/RUNB encoded is equal to the block size (post
+                  the initial RLE), viz, 900k, so bounding N at 2
+                  million should guard against overflow without
+                  rejecting any legitimate inputs. */
+               if (N >= 2*1024*1024) RETURN(BZ_DATA_ERROR);
                if (nextSym == BZ_RUNA) es = es + (0+1) * N; else
                if (nextSym == BZ_RUNB) es = es + (1+1) * N;
                N = N * 2;