7568 – pragma(msg) segfaults with an aggregate including a class.

Test cases:
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class C7568 {}
struct S7568 { C7568 c; }
auto test7568a() { return [new C7568]; }
auto test7568b() { return S7568(new C7568); }
pragma(msg, test7568a());
pragma(msg, test7568b());
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This will cause a segmentation fault:
 test.d(4): Error: C7568 class literals cannot be returned from CTFE
 Segmentation fault
This happens because an Expression* is assigned to the value 0x1, and the function 'argsToCBuffer' is trying to access the expression without first checking if the expression is a valid pointer.
The program should simply fail to compile, without the segfault.

https://github.com/D-Programming-Language/dmd/pull/774 

Commit pushed to dmd-1.x at https://github.com/D-Programming-Language/dmd
https://github.com/D-Programming-Language/dmd/commit/8fadb858512dda3f2c697a6b55bb5d2369c18995
fix Issue 7568 - pragma(msg) segfaults with an aggregate including a class.