Fix memory allocation failed regression introduced by #22766

[ibuclaw]

When Object doesn't exist, there is no __monitor field to skip.

core.exception.OutOfMemoryError@src/dmd/root/rmem.d(103): Memory allocation failed
----------------
??:? onOutOfMemoryError [0x5a5c859d445c]
src/dmd/root/rmem.d:103 _ZN3Mem5errorEv [0x5a5c858a316b]
src/dmd/root/rmem.d:117 _ZN3Mem5checkEPv [0x5a5c858a3189]
src/dmd/root/rmem.d:46 _ZN3Mem7xmallocEm [0x5a5c858a2f76]
/home/ibuclaw/src/dlang/dmd/compiler/src/dmd/root/array.d:182 pure nothrow void dmd.root.array.Array!(dmd.expression.Expression).Array.reserve(ulong).enlarge(ulong) [0x5a5c8558fd8a]
/home/ibuclaw/src/dlang/dmd/compiler/src/dmd/root/array.d:239 _ZN5ArrayIP10ExpressionE7reserveEm [0x5a5c8558fd35]
/home/ibuclaw/src/dlang/dmd/compiler/src/dmd/root/array.d:46 _ZN5ArrayIP10ExpressionEC1Em [0x5a5c8558fb33]
src/dmd/dinterpret.d:2873 _ZN11Interpreter5visitEP6NewExp [0x5a5c8566b068]