scripts: protect against partially extracted source

In case of failure (eg. user hitting Ctrl-C), avoid extracting over
partially extracted source. Will help once we have git available.

 reporter_name="unknown"

 reporter_url="http://iphonelinux.org/index.php/Glibc_toolchain_using_crosstool-NG"

 reporter_comment="An (experimental) toolchain targeting the Apple's iPhone."