loader.c: Fix possible SEGV when a module fails to register

When a module fails to register itself (usually a coding error
in the module), dlerror() can return NULL.  We weren't checking
for that in load_dlopen() before trying to strdup the error message
so a SEGV was thrown.  dlerror() is now surrounded with an S_OR
so we don't SEGV.

Change-Id: Ie0fb9316f08a321434f3f85aecf3c7d2ede8b956