Fix leak

Introduced by yours-truly in 7212c3573e7f2b13f907e17a5683d382b34ab932

Oopsie.
auto merge of #2607 : bjz/servo/leeeeeak, r=jdm

Introduced by yours-truly in 7212c3573e7f2b13f907e17a5683d382b34ab932

Oopsie.