Fix misplaced show_line attribute

Under the current position, show_line is assigned with a location
object and causes:

```
❯ exe/rdbg -e  'trace on;; quit!' target.rb
<NoMethodError: undefined method `+' for "target.rb:1:in `<main>'":Thread::Backtrace::Location>
["/Users/st0012/projects/debug/lib/debug/thread_client.rb:257:in `show_src'",
 "/Users/st0012/projects/debug/lib/debug/thread_client.rb:163:in `on_suspend'",
 "/Users/st0012/projects/debug/lib/debug/thread_client.rb:140:in `on_breakpoint'",
 "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:50:in `suspend'",
 "/Users/st0012/projects/debug/lib/debug/breakpoint.rb:102:in `block in setup'",
 "target.rb:1:in `<main>'"]
```

lib/debug/frame_info.rb

@@ -1,9 +1,8 @@
 
 module DEBUGGER__
   FrameInfo = Struct.new(:location, :self, :binding, :iseq, :class, :frame_depth,
-                          :has_return_value, :return_value,
-                          :has_raised_exception, :raised_exception,
-                          :show_line)
+                          :has_return_value, :return_value, :show_line,
+                          :has_raised_exception, :raised_exception)
 
   # extend FrameInfo with debug.so
   if File.exist? File.join(__dir__, 'debug.so')