fix unused variable warning

Author: beepster4096

src/shims/backtrace.rs

@@ -243,7 +243,7 @@ pub trait EvalContextExt<'mir, 'tcx: 'mir>: crate::MiriEvalContextExt<'mir, 'tcx
             throw_unsup_format!("unknown `miri_backtrace_size` flags {}", flags);
         }
 
-        let (fn_instance, lo, name, filename) = this.resolve_frame_pointer(ptr)?;
+        let (_, _, name, filename) = this.resolve_frame_pointer(ptr)?;
 
         this.memory.write_bytes(this.read_pointer(name_ptr)?, name.bytes())?;
         this.memory.write_bytes(this.read_pointer(filename_ptr)?, filename.bytes())?;