Add test case for #1320

From https://github.com/cc65/cc65/issues/1320#issuecomment-726866015

test/err/bug1320.c

@@ -0,0 +1,35 @@
+/*
+  Copyright 2020 The cc65 Authors
+
+  This software is provided 'as-is', without any express or implied
+  warranty. In no event will the authors be held liable for any damages
+  arising from the use of this software.
+
+  Permission is granted to anyone to use this software for any purpose,
+  including commercial applications, and to alter it and redistribute it
+  freely, subject to the following restrictions:
+
+  1. The origin of this software must not be misrepresented; you must not
+     claim that you wrote the original software. If you use this software
+     in a product, an acknowledgment in the product documentation would be
+     appreciated but is not required.
+  2. Altered source versions must be plainly marked as such, and must not be
+     misrepresented as being the original software.
+  3. This notice may not be removed or altered from any source distribution.
+*/
+
+/*
+  Test of access violation.
+  https://github.com/cc65/cc65/issues/1320
+  This should compile and should be moved to tests/val/ when the bug is fixed.
+*/
+
+char *var;
+
+void foo (char, char);
+char bar (void);
+
+void main (void)
+{
+    foo (*var++, bar ());
+}