Don't evaluate arbitrary text coming from SYMPY_DEBUG variable

With all the talks about recent shellshock vulnerability discovery in
Bash I've remembered encountering similar unguarded execution of
arbitrary text coming from the environment in sympy.  It is not that
critical, but it may be used to gain root privileges if there's a
script importing sympy with setuid bit on it:

```
$ SYMPY_DEBUG='os.sys.stdout.write("Hello\n")' python -c 'import sympy'
Hello
Hello
```

This PR should fix the issue.

I'm not sure where to put tests for it, though, as there's no test
suite in the root directory, maybe into `sympy.core`?
On Wed, Oct 01, 2014 at 09:16:39AM -0700, immerrr again wrote:
>    Is winning three lines of vertical space worth adding an eval? (for me
>    you always need

You always need to know what you are doing.  eval is not
an evil: it's just another, well documented function.