HyperLogLog regression test for issue #1762.

tests/unit/hyperloglog.tcl

@@ -156,4 +156,15 @@ start_server {tags {"hll"}} {
         r pfadd hll 1 2 3
         llength [r pfdebug getreg hll]
     } {16384}
+
+    test {PFADD / PFCOUNT cache invalidation works} {
+        r del hll
+        r pfadd hll a b c
+        r pfcount hll
+        assert {[r getrange hll 15 15] eq "\x00"}
+        r pfadd hll a b c
+        assert {[r getrange hll 15 15] eq "\x00"}
+        r pfadd hll 1 2 3
+        assert {[r getrange hll 15 15] eq "\x80"}
+    }
 }