object_path: fix potential buffer overrun

The `filename` function attempted to look at the first character
of a substring, which could have been empty.  This was noticed by
UBSAN in GCC 13.2.

Signed-off-by: Patrick Williams <[email protected]>
Change-Id: I7397861b6b19ef86d1c5c01dc1007a8804d08ab6

Author: williamspatrick

src/message/native_types.cpp

@@ -64,7 +64,7 @@ std::string string_path_wrapper::filename() const
 
     // If we don't see that this was encoded by sdbusplus, return the naive
     // version of the filename path.
-    if (filename[0] != '_')
+    if (filename.empty() || filename[0] != '_')
     {
         return std::string(filename);
     }