go/types: don't crash when assigning to undefined variables

R=adonovan
CC=golang-dev
https://golang.org/cl/7369059

src/pkg/go/types/stmt.go

@@ -62,6 +62,10 @@ func (check *checker) assign1to1(lhs, rhs ast.Expr, x *operand, decl bool, iota
 			}
 		}
 
+		if x.mode == invalid || z.mode == invalid {
+			return
+		}
+
 		check.assignOperand(&z, x)
 		if x.mode != invalid && z.mode == constant {
 			check.errorf(x.pos(), "cannot assign %s to %s", x, &z)

src/pkg/go/types/testdata/stmt0.src

@@ -32,6 +32,8 @@ func _() {
 
 	var u64 uint64
 	u64 += 1<<u64
+
+	undeclared /* ERROR "undeclared" */ = 991
 }
 
 func _incdecs() {