Check spawn_func() return value

If we don't, we can wind up with a wedged cpuworker, and write to it
for ages and ages.

Found by skruffy.  This was a bug in 2dda97e8fd898757, a.k.a. svn
revision 402. It's been there since we have been using cpuworkers.

changes/bug4345

@@ -0,0 +1,6 @@
+  o Minor bugfixes:
+    - Check return code on spawn_func() in cpuworker code, so that we don't
+      think we've spawned a nonworking cpuworker and write junk to it
+      forever. Fix for bug 4345; bugfix on all released Tor versions.
+      Found by "skruffy".
+

src/or/cpuworker.c

@@ -528,7 +528,12 @@ spawn_cpuworker(void)
   tor_assert(SOCKET_OK(fdarray[1]));
 
   fd = fdarray[0];
-  spawn_func(cpuworker_main, (void*)fdarray);
+  if (spawn_func(cpuworker_main, (void*)fdarray) < 0) {
+    tor_close_socket(fdarray[0]);
+    tor_close_socket(fdarray[1]);
+    tor_free(fdarray);
+    return -1;
+  }
   log_debug(LD_OR,"just spawned a cpu worker.");
 #ifndef TOR_IS_MULTITHREADED
   tor_close_socket(fdarray[1]); /* don't need the worker's side of the pipe */