interp: check that send operate on channel value

Not performing this check was leading to a panic at run-time. It now fails early with a compile error.

Fixes #1453.

interp/cfg.go

@@ -960,7 +960,14 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 			}
 			wireChild(n)
 
-		case declStmt, exprStmt, sendStmt:
+		case sendStmt:
+			if !isChan(n.child[0].typ) {
+				err = n.cfgErrorf("invalid operation: cannot send to non-channel %s", n.child[0].typ.id())
+				break
+			}
+			fallthrough
+
+		case declStmt, exprStmt:
 			wireChild(n)
 			l := n.lastChild()
 			n.findex = l.findex

interp/interp_eval_test.go

@@ -642,6 +642,8 @@ func TestEvalChan(t *testing.T) {
 				return ok && msg == "ping"
 			})()`, res: "true",
 		},
+		{src: `a :=5; a <- 4`, err: "cannot send to non-channel int"},
+		{src: `a :=5; b := <-a`, err: "cannot receive from non-channel int"},
 	})
 }