interp: fix mismatch assign statement panic

Follow by the [Spec](https://go.dev/ref/spec#Assignment_statements):

The number of operands on the left hand side must match the number of values. For instance, if f is a function returning two values `x, y = f()` assigns the first value to x and the second to y.

In the second form, the number of operands on the left must equal the number of expressions on the right, each of which must be single-valued, and the nth expression on the right is assigned to the nth operand on the left.

Fixes traefik#1606

_test/assign19.go

@@ -0,0 +1,9 @@
+package main
+
+func main() {
+	a, b, c := 1, 2
+	_, _, _ = a, b, c
+}
+
+// Error:
+// _test/assign19.go:4:2: cannot assign 2 values to 3 variables

interp/cfg.go

@@ -647,6 +647,14 @@ func (interp *Interpreter) cfg(root *node, sc *scope, importPath, pkgName string
 				sbase = len(n.child) - n.nright
 			}
 
+			// If len(RHS) > 1, each node must be single-valued, and the nth expression
+			// on the right is assigned to the nth operand on the left, so the number of
+			// nodes on the left and right sides must be equal
+			if n.nright > 1 && n.nright != n.nleft {
+				err = n.cfgErrorf("cannot assign %d values to %d variables", n.nright, n.nleft)
+				return
+			}
+
 			wireChild(n)
 			for i := 0; i < n.nleft; i++ {
 				dest, src := n.child[i], n.child[sbase+i]

interp/interp_consistent_test.go

@@ -37,6 +37,7 @@ func TestInterpConsistencyBuild(t *testing.T) {
 			file.Name() == "assign11.go" || // expect error
 			file.Name() == "assign12.go" || // expect error
 			file.Name() == "assign15.go" || // expect error
+			file.Name() == "assign19.go" || // expect error
 			file.Name() == "bad0.go" || // expect error
 			file.Name() == "break0.go" || // expect error
 			file.Name() == "cont3.go" || // expect error