interp: fix parsing of assign to dereferenced pointer

Fixes #969.

interp/cfg.go

@@ -75,6 +75,9 @@ func (interp *Interpreter) cfg(root *node, importPath string) ([]*node, error) {
 			case assignStmt, defineStmt:
 				a := n.anc
 				i := childPos(n) - a.nright
+				if i < 0 {
+					break
+				}
 				if len(a.child) > a.nright+a.nleft {
 					i--
 				}

interp/interp_eval_test.go

@@ -110,6 +110,7 @@ func TestEvalAssign(t *testing.T) {
 		{src: "f := int64(3.2)", err: "1:39: cannot convert expression of type float64 to type int64"},
 		{src: "g := 1; g <<= 8", res: "256"},
 		{src: "h := 1; h >>= 8", res: "0"},
+		{src: "i := 1; j := &i; (*j) = 2", res: "2"},
 	})
 }