crash_generation: fix bad call to close

If signal_fd is -1 still, we end up calling close(-1).  Not generally
a problem, but it's bad form, and coverity is upset by it.

Change-Id: I46f9c7ca4be7b43af5b609dd8e3f03a0700af418
Reviewed-on: https://chromium-review.googlesource.com/414544
Reviewed-by: Primiano Tucci <primiano@chromium.org>

src/client/linux/crash_generation/crash_generation_server.cc

@@ -255,7 +255,7 @@ CrashGenerationServer::ClientEvent(short revents)
   }
 
   if (crashing_pid == -1 || signal_fd == -1) {
-    if (signal_fd)
+    if (signal_fd != -1)
       close(signal_fd);
     return true;
   }