[LLVM/ABI] Don't pass null pointer to byVal attribute

src/abi_x86_64.cpp

@@ -202,7 +202,6 @@ bool needPassByRef(jl_datatype_t *dt, AttrBuilder &ab, LLVMContext &ctx, Type *T
     else if (jl_is_structtype(dt)) {
         // spill to memory even though we would ordinarily pass
         // it in registers
-        Type* Ty = preferred_llvm_type(dt, false, ctx);
         ab.addByValAttr(Ty);
         return true;
     }

src/ccall.cpp

@@ -1098,6 +1098,7 @@ std::string generate_func_sig(const char *fname)
         }
 
         // Whether or not LLVM wants us to emit a pointer to the data
+        assert(t && "LLVM type should not be null");
         bool byRef = abi->needPassByRef((jl_datatype_t*)tti, ab, LLVMCtx, t);
 
         if (jl_is_cpointer_type(tti)) {