Add a DCHECK that ArenaStringPtr::Set(char*, Arena*) is not called with

a null char* argument.

This is already undefined behavior since the argument is immediately
used to construct a string_view, and passing a null argument to the
single-arg string_view constructor is undefined behavior. Adding the
check will prevent introduction of new bugs when using standard
library implementations that do not detect this case.

PiperOrigin-RevId: 787178436

Author: derekmauro

src/google/protobuf/arenastring.h

@@ -462,6 +462,7 @@ inline void ArenaStringPtr::InitAllocated(std::string* str, Arena* arena) {
 }
 
 inline void ArenaStringPtr::Set(const char* s, Arena* arena) {
+  ABSL_DCHECK(s != nullptr);
   Set(absl::string_view{s}, arena);
 }