Fix fastq downsampling.
Fix fastq downsampling.

If the first element of rand_records is high, there is a chance that it will move to the end in the first iteration of the while loop, and write only 1 read. The rest N-1 readlines will return an empty line.
Cyberbio-Lab/bcbio-nextgen,vladsaveliev/bcbio-nextgen,biocyberman/bcbio-nextgen,chapmanb/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen,brainstorm/bcbio-nextgen,vladsaveliev/bcbio-nextgen,mjafin/bcbio-nextgen,a113n/bcbio-nextgen,mjafin/bcbio-nextgen,lbeltrame/bcbio-nextgen,brainstorm/bcbio-nextgen,biocyberman/bcbio-nextgen,a113n/bcbio-nextgen,mjafin/bcbio-nextgen,lbeltrame/bcbio-nextgen,chapmanb/bcbio-nextgen,brainstorm/bcbio-nextgen,vladsaveliev/bcbio-nextgen,chapmanb/bcbio-nextgen,lbeltrame/bcbio-nextgen,a113n/bcbio-nextgen,biocyberman/bcbio-nextgen,Cyberbio-Lab/bcbio-nextgen