compiler: do not DROP unary expression value inside IF stmt

We dropped values from such calls because they where marked as unused
(consequently, were followed by DROP instructions). Example:

if !foo() {    <--- panic here
    ...
}

This commit prevents the following runtime error during script execution:
```
"error encountered at instruction ** (NOT): runtime error: invalid memory address or nil pointer dereference"

```

pkg/compiler/func_scope.go

@@ -112,6 +112,11 @@ func (c *funcScope) analyzeVoidCalls(node ast.Node) bool {
 		if ok {
 			c.voidCalls[ce] = false
 		}
+	case *ast.UnaryExpr:
+		ce, ok := n.X.(*ast.CallExpr)
+		if ok {
+			c.voidCalls[ce] = false
+		}
 	case *ast.IfStmt:
 		// we can't just return `false`, because we still need to process body
 		ce, ok := n.Cond.(*ast.CallExpr)

pkg/compiler/if_test.go

@@ -121,3 +121,32 @@ func TestInitIF(t *testing.T) {
 		})
 	})
 }
+
+func TestCallExpIF(t *testing.T) {
+	t.Run("Call", func(t *testing.T) {
+		src := `package foo
+			func someFunc() bool {
+				return true
+			}
+			func Main() int {
+				if someFunc() {
+					return 5
+				}
+				return 6
+			}`
+		eval(t, src, big.NewInt(5))
+	})
+	t.Run("CallWithUnaryExpression", func(t *testing.T) {
+		src := `package foo
+			func someFunc() bool {
+				return false
+			}
+			func Main() int {
+				if !someFunc() {
+					return 5
+				}
+				return 6
+			}`
+		eval(t, src, big.NewInt(5))
+	})
+}