Fix hstore binary null decoding

Bug was advancing the read pointer by the length of the value even if it
was a NULL value. Since NULL is indicated by a -1 length it actually
decremented the read pointer.

hstore.go

@@ -142,8 +142,8 @@ func (dst *Hstore) DecodeBinary(ci *ConnInfo, src []byte) error {
 		var valueBuf []byte
 		if valueLen >= 0 {
 			valueBuf = src[rp : rp+valueLen]
+			rp += valueLen
 		}
-		rp += valueLen
 
 		var value Text
 		err := value.DecodeBinary(ci, valueBuf)

hstore_test.go

@@ -21,6 +21,10 @@ func TestHstoreTranscode(t *testing.T) {
 		&pgtype.Hstore{Map: map[string]pgtype.Text{"NULL": text("bar")}, Status: pgtype.Present},
 		&pgtype.Hstore{Map: map[string]pgtype.Text{"foo": text("NULL")}, Status: pgtype.Present},
 		&pgtype.Hstore{Map: map[string]pgtype.Text{"": text("bar")}, Status: pgtype.Present},
+		&pgtype.Hstore{
+			Map:    map[string]pgtype.Text{"a": text("a"), "b": {Status: pgtype.Null}, "c": text("c"), "d": {Status: pgtype.Null}, "e": text("e")},
+			Status: pgtype.Present,
+		},
 		&pgtype.Hstore{Status: pgtype.Null},
 	}