catch the case where there is no symCmd

We do check for symCmd, to set the info->nlist value, but forgot to do
the same check for info->names.  Thus when trying to extract stroff from
symCmd, we hit a segfault.

Test Plan: The validation failure on windows is rather suspicious...
let's try this one

Reviewers: bgamari, adinapoli, austin, erikd, simonmar

Reviewed By: adinapoli

Subscribers: thomie, rwbarton

Differential Revision: https://phabricator.haskell.org/D3468