Fixed a segfault when compaction fails

Summary: We've hit it today.

Test Plan: `make -j check`; didn't reproduce the issue

Reviewers: yhchiang

Reviewed By: yhchiang

Subscribers: dhruba

Differential Revision: https://reviews.facebook.net/D54219

db/db_impl.cc

@@ -1849,7 +1849,6 @@ Status DBImpl::CompactFilesImpl(
         c->column_family_data(), job_context, *c->mutable_cf_options());
   }
   c->ReleaseCompactionFiles(s);
-  c.reset();
 
   if (status.ok()) {
     // Done
@@ -1865,6 +1864,8 @@ Status DBImpl::CompactFilesImpl(
     }
   }
 
+  c.reset();
+
   bg_compaction_scheduled_--;
   if (bg_compaction_scheduled_ == 0) {
     bg_cv_.SignalAll();