dircache: don't unpin invalid buflib handle when building volumes

Buflib pinning was introduced here in 1718cf5.

If the device has to immediately shut down after booting
because of a low battery, the directory cache may be disabled
by the time core_unpin() is reached.

Its handle will have been reset to 0 in that case, triggering
a panic message due to an invalid buflib handle.

Change-Id: Ie34c3f7142d8406b66277ef185bc9ecf387d7295

firmware/common/dircache.c

@@ -1882,6 +1882,7 @@ static void build_volumes(void)
 
     logf("Done, %ld KiB used", dircache.size / 1024);
 
+    if (dircache_runinfo.handle > 0) /* dircache may have been disabled */
         core_unpin(dircache_runinfo.handle);
 }