scram-project-build: Check if debug logs exist before compressing them

When I tried to build fwlite on osx108_amd64_gcc481 I had the following error: 

<pre>
+ pushd /Volumes/build1/dmendezl/CMSSW_7_1_6-build/tmp/BUILDROOT/a36aed06553e54d6eb76ebc2a5f45550/opt/cmssw/osx108_amd64_gcc481/cms/fwlite/CMSSW_7_1_6_FWLITE/tmp/osx108_amd64_gcc481/cache/log/src
/Volumes/build1/dmendezl/CMSSW_7_1_6-build/tmp/rpm-tmp.kliX35: line 66: pushd: /Volumes/build1/dmendezl/CMSSW_7_1_6-build/tmp/BUILDROOT/a36aed06553e54d6eb76ebc2a5f45550/opt/cmssw/osx108_amd64_gcc481/cms/fwlite/CMSSW_7_1_6_FWLITE/tmp/osx108_amd64_gcc481/cache/log/src: No such file or directory
error: Bad exit status from /Volumes/build1/dmendezl/CMSSW_7_1_6-build/tmp/rpm-tmp.kliX35 (%build)
</pre>

I used this workaround to make it work.