gh-158063: Fix out-of-bounds read when comparing decimal signal dicts

[lazerg]

signaldict_richcompare() checked SdFlagAddr(w) before it knew w was a signal dict, so comparing flags or traps with anything else (e.g. None) read the flags pointer from past the end of that object. The NULL check on w now only runs when w is a signal dict. Other objects fall through to the existing dict/NotImplemented handling.

• Issue: AddressSanitizer: global-buffer-overflow Modules/_decimal/_decimal.c:798:37 in signaldict_richcompare #158063