[brettwooldridge#15] previousSetBit resets word length after miss

Currently it sets the length to the position of the needle in the first
word it looks up and only searches indices below that. This is incorrect
after the initial word where we need to search from the top.

src/main/java/com/zaxxer/sparsebits/SparseBitSet.java

@@ -1044,7 +1044,7 @@ public int previousClearBit(int i)
         if (w1 > aLength - 1)
             return i;
         w1 = Math.min(w1, aLength - 1);
-        final int w4 = i % LENGTH4;
+        int w4 = i % LENGTH4;
 
         long word;
         long[][] a2;
@@ -1074,6 +1074,7 @@ public int previousClearBit(int i)
                         if ((word & (1L << bitIdx)) == 0)
                             return (((w1 << SHIFT1) + (w2 << SHIFT2) + w3) << SHIFT3) + bitIdx;
                     }
+                    w4 = LENGTH4 - 1;
                 }
                 w3 = LENGTH3 - 1;
             }

src/test/java/com/zaxxer/sparsebits/PreviousClearBitTest.java

@@ -206,4 +206,14 @@ public void randomMultiEntry() throws Exception {
             values.clear();
         }
     }
+
+    @Test
+    public void bug15() throws Exception {
+        set.set(1);
+        set.set(64);
+        assertEquals(63, set.previousClearBit(64));
+        set.clear(0);
+        set.set(1);
+        assertEquals(63, set.previousClearBit(64));
+    }
 }