Fix a NULL deref in tcp_client_windows.cc
Fix a NULL deref in tcp_client_windows.cc

`grpc_sockaddr_to_uri(addr)` can return nullptr, and we are directly
passing it to grpc_slice_from_copied_string. Clusterfuzz found this
issue in https://clusterfuzz.com/testcase-detail/5188592759603200

Use "NULL" when target URI is nullptr, to avoid null deref.

Fixes 18544

nicolasnoble/grpc,donnadionne/grpc,donnadionne/grpc,pszemus/grpc,pszemus/grpc,jboeuf/grpc,jboeuf/grpc,ejona86/grpc,grpc/grpc,stanley-cheung/grpc,ctiller/grpc,pszemus/grpc,vjpai/grpc,donnadionne/grpc,nicolasnoble/grpc,jboeuf/grpc,ejona86/grpc,ejona86/grpc,jboeuf/grpc,vjpai/grpc,vjpai/grpc,jtattermusch/grpc,stanley-cheung/grpc,pszemus/grpc,firebase/grpc,pszemus/grpc,muxi/grpc,pszemus/grpc,ejona86/grpc,jboeuf/grpc,grpc/grpc,ejona86/grpc,nicolasnoble/grpc,muxi/grpc,stanley-cheung/grpc,muxi/grpc,muxi/grpc,vjpai/grpc,sreecha/grpc,stanley-cheung/grpc,ctiller/grpc,jboeuf/grpc,sreecha/grpc,ejona86/grpc,ctiller/grpc,jtattermusch/grpc,nicolasnoble/grpc,muxi/grpc,nicolasnoble/grpc,donnadionne/grpc,firebase/grpc,donnadionne/grpc,donnadionne/grpc,firebase/grpc,sreecha/grpc,pszemus/grpc,stanley-cheung/grpc,muxi/grpc,stanley-cheung/grpc,grpc/grpc,donnadionne/grpc,grpc/grpc,nicolasnoble/grpc,ejona86/grpc,vjpai/grpc,donnadionne/grpc,jboeuf/grpc,firebase/grpc,ejona86/grpc,firebase/grpc,ctiller/grpc,sreecha/grpc,vjpai/grpc,grpc/grpc,sreecha/grpc,jboeuf/grpc,sreecha/grpc,ctiller/grpc,jboeuf/grpc,stanley-cheung/grpc,donnadionne/grpc,jboeuf/grpc,vjpai/grpc,ejona86/grpc,sreecha/grpc,jtattermusch/grpc,pszemus/grpc,muxi/grpc,vjpai/grpc,jboeuf/grpc,ejona86/grpc,jtattermusch/grpc,nicolasnoble/grpc,muxi/grpc,jtattermusch/grpc,pszemus/grpc,firebase/grpc,vjpai/grpc,firebase/grpc,pszemus/grpc,grpc/grpc,ctiller/grpc,stanley-cheung/grpc,muxi/grpc,muxi/grpc,ctiller/grpc,vjpai/grpc,stanley-cheung/grpc,donnadionne/grpc,ctiller/grpc,jtattermusch/grpc,nicolasnoble/grpc,stanley-cheung/grpc,firebase/grpc,stanley-cheung/grpc,firebase/grpc,jtattermusch/grpc,grpc/grpc,ctiller/grpc,firebase/grpc,jtattermusch/grpc,jtattermusch/grpc,grpc/grpc,firebase/grpc,nicolasnoble/grpc,jtattermusch/grpc,sreecha/grpc,grpc/grpc,nicolasnoble/grpc,vjpai/grpc,pszemus/grpc,grpc/grpc,stanley-cheung/grpc,sreecha/grpc,grpc/grpc,sreecha/grpc,donnadionne/grpc,pszemus/grpc,nicolasnoble/grpc,ctiller/grpc,nicolasnoble/grpc,jtattermusch/grpc,vjpai/grpc,grpc/grpc,muxi/grpc,jtattermusch/grpc,ejona86/grpc,donnadionne/grpc,jboeuf/grpc,muxi/grpc,ctiller/grpc,sreecha/grpc,ctiller/grpc,sreecha/grpc,ejona86/grpc,firebase/grpc