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.
```

1 files changed, 1 insertions, 1 deletions

diff --git a/mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c b/mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c
index 9759badfe..d4ec3d5f8 100755
--- a/mrbgems/mruby-bin-debugger/tools/mrdb/cmdbreak.c
+++ b/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 {