fix crash in pre4 Assertion `seedCollection==nullptr' failed.

seedCollection needs to be reset to zero on a legal exit from the method, otherwise next call will give an assert

should fix crashes reported in https://hypernews.cern.ch/HyperNews/CMS/get/relval/3121.html
Tested with  http://cms-project-relval.web.cern.ch/cms-project-relval/failures/CMSSW_7_2_0_pre4/standard/WElSkim2012D__RelVal_wEl2012D_Job1.tar.gz