It is not possible to sort by stars when searching inside code.
From an e-mail from github.com support:
Code search does not support sorting by number of stars, but I will definitely add your +1 to that suggestion internally! I can’t say if or when a change will happen, but your feedback is in the right hands.
You may want to vote for this feature: Add Stars count filter.
The feedback is related to this announcement: Improving GitHub code search