stored: Show the output of dlerror() correctly.

As we use NPRT(dlerror()) we call dlerror() twice while expanding
the NPRT() macro and the manual page of dlerror() says:

The returned string contains no trailing newline. If no dynamic
linking errors have occurred since the last invocation of dlerror(),
dlerror() returns NULL. Thus, invoking  dlerror() a second time,
immediately following a prior invocation, results in NULL being
returned.

src/stored/sd_backends.c

@@ -73,6 +73,7 @@ DEVICE *init_backend_dev(JCR *jcr, int device_type)
    char *backend_dir;
    void *dl_handle = NULL;
    POOL_MEM shared_library_name(PM_FNAME);
+   POOL_MEM error(PM_FNAME);
    backend_interface_mapping_t *backend_interface_mapping;
    backend_shared_library_t *backend_shared_library;
    t_backend_instantiate backend_instantiate;
@@ -117,8 +118,11 @@ DEVICE *init_backend_dev(JCR *jcr, int device_type)
       if (stat(shared_library_name.c_str(), &st) == 0) {
          dl_handle = dlopen(shared_library_name.c_str(), RTLD_NOW);
          if (!dl_handle) {
+            pm_strcpy(error, dlerror());
             Jmsg(jcr, M_ERROR, 0, _("Unable to load shared library: %s ERR=%s\n"),
-                 shared_library_name.c_str(), NPRT(dlerror()));
+                 shared_library_name.c_str(), error.c_str());
+            Dmsg2(100, _("Unable to load shared library: %s ERR=%s\n"),
+                  shared_library_name.c_str(), error.c_str());
             continue;
          }
 
@@ -127,8 +131,11 @@ DEVICE *init_backend_dev(JCR *jcr, int device_type)
           */
          backend_instantiate = (t_backend_instantiate)dlsym(dl_handle, "backend_instantiate");
          if (backend_instantiate == NULL) {
+            pm_strcpy(error, dlerror());
             Jmsg(jcr, M_ERROR, 0, _("Lookup of backend_instantiate in shared library %s failed: ERR=%s\n"),
-                 shared_library_name.c_str(), NPRT(dlerror()));
+                 shared_library_name.c_str(), error.c_str());
+            Dmsg2(100, _("Lookup of backend_instantiate in shared library %s failed: ERR=%s\n"),
+                  shared_library_name.c_str(), error.c_str());
             dlclose(dl_handle);
             dl_handle = NULL;
             continue;
@@ -139,8 +146,11 @@ DEVICE *init_backend_dev(JCR *jcr, int device_type)
           */
          flush_backend = (t_flush_backend)dlsym(dl_handle, "flush_backend");
          if (flush_backend == NULL) {
+            pm_strcpy(error, dlerror());
             Jmsg(jcr, M_ERROR, 0, _("Lookup of flush_backend in shared library %s failed: ERR=%s\n"),
-                 shared_library_name.c_str(), NPRT(dlerror()));
+                 shared_library_name.c_str(), error.c_str());
+            Dmsg2(100, _("Lookup of flush_backend in shared library %s failed: ERR=%s\n"),
+                  shared_library_name.c_str(), error.c_str());
             dlclose(dl_handle);
             dl_handle = NULL;
             continue;