cmd/compile: use correct variable when setting up dummy CallStmt in error

Fixes crash when printing a related error message later on.

Fixes #20789.

Change-Id: I6d2c35aafcaeda26a211fc6c8b7dfe4a095a3efe
Reviewed-on: https://go-review.googlesource.com/46713
Run-TryBot: Robert Griesemer <gri@golang.org>
Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Reviewed-by: Matthew Dempsky <mdempsky@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/compile/internal/syntax/parser.go

@@ -654,7 +654,7 @@ func (p *parser) callStmt() *CallStmt {
 	if !ok {
 		p.error(fmt.Sprintf("expression in %s must be function call", s.Tok))
 		// already progressed, no need to advance
-		cx := new(CallExpr)
+		cx = new(CallExpr)
 		cx.pos = x.Pos()
 		cx.Fun = p.bad()
 	}