The gimple_call_alloc_size() function is documented to "return null
when STMT is not a call to a valid allocation function" but the code
assumes STMT is a call statement, causing the function to ICE when
it isn't.

The attached patch changes the function to fulfill its contract and
return null also when STMT isn't a call.  The fix seems obvious to
me but I'll wait some time before committing it in case it's not
to someone else.

Martin