Make sure to consider small distances in LZ77.

This could corrupt certain images since commit
a3611513d2

Change-Id: Ifbe43abaafe8efb27c62af18039fea5a9dc4e062

src/enc/backward_references.c

@@ -464,7 +464,7 @@ static int BackwardReferencesLz77(int xsize, int ysize,
         }
       }
     } else {
-      len = 1;
+      len = (len == 0) ? 1 : len;
     }
     // Go with literal or backward reference.
     assert(len > 0);