Fix: GPv3: Crash renaming a layer

When renaming a layer to a (long) name that was already taken
by another layer, Blender would crash.
This was because the memory for the unique string was held by
`std::string` which cannot grow.
The solution is to use `char unique_name[MAX_NAME]`.

source/blender/blenkernel/intern/grease_pencil.cc

@@ -2014,9 +2014,10 @@ static std::string unique_node_name(const GreasePencil &grease_pencil,
                                     blender::StringRefNull name)
 {
   using namespace blender;
-  std::string unique_name(name.c_str());
+  char unique_name[MAX_NAME];
+  BLI_strncpy(unique_name, name.c_str(), MAX_NAME);
   VectorSet<StringRefNull> names = get_node_names(grease_pencil);
-  unique_node_name_ex(names, default_name, unique_name.data());
+  unique_node_name_ex(names, default_name, unique_name);
   return unique_name;
 }