Reset underscore count to zero with intervening digit

We're only supposed to bail out on 2 or more consecutive
underscores. Probably ported wrong since us += 0 makes 0 sense.

Fixes jruby#7749

core/src/main/java/org/jruby/util/ConvertBytes.java

@@ -558,7 +558,7 @@ private void squeezeZeroes() {
                         break;
                     }
                 } else {
-                    us += 0;
+                    us = 0;
                 }
                 beg++;
             }