Make the null pointer dereference a volatile one

This is to ensure that the compiler won't optimise it out of existence.
Clang says it will do it:

testProcessCrash/main.cpp:50:5: warning: indirection of non-volatile null pointer will be deleted, not trap [-Wnull-dereference]
    *(char*)0 = 0;
    ^~~~~~~~~

Change-Id: Iac7771046442f869e205e8789fffdd6443d58e67
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>

tests/auto/corelib/io/qprocess/testProcessCrash/main.cpp

@@ -47,7 +47,7 @@ struct Foo
 
 int main()
 {
-    *(char*)0 = 0;
+    *(volatile char*)0 = 0;
     Foo *f = 0;
     return f->i;
 }