SIGTRAP leaks when using backquotes and object allocation profiler

The whole interpreter aborts with SIGTRAP when using the object allocation profiler. Simply creating the following bla.rb file

```
`ls`
```

and running

```
CPUPROFILE_OBJECTS=1 CPUPROFILE=acd-profile-allocs ruby -rperftools -S bla.rb
```

triggers it on my machine (Debian unstable, ruby version)

```
ruby 1.9.3p194 (2012-04-20 revision 35410) [x86_64-linux]
```
