mrdb: fix that break command cannot handle Windows paths

Before fix:
```
$ mrdb c:\tmp\b.rb
(c:\tmp\b.rb:1) break c:\tmp\b.rb:3
Class name 'c' is invalid.
```

After fix:
```
$ mrdb c:\tmp\b.rb
(c:\tmp\b.rb:1) break c:\tmp\b.rb:3
Breakpoint 1: file c:\tmp\b.rb, line 3.
```

mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c

@@ -256,7 +256,7 @@ parse_breakcommand(mrdb_state *mrdb, const char **file, uint32_t *line, char **c
   }
 
   args = mrdb->words[1];
-  if((body = strchr(args, ':')) == NULL) {
+  if((body = strrchr(args, ':')) == NULL) {
     body = args;
     type = check_bptype(body);
   } else {