DynamicLoader: Fix CloseLibrary for failed OpenLibrary on HP-UX

Suggested-by: Gerhard Grimm <gerhard.grimm@detec.com>
Change-Id: I22bf9a8e14c5c3862f6422ec2d8cc4da645a5a0d

DynamicLoader.cxx

@@ -48,6 +48,10 @@ DynamicLoader::LibraryHandle DynamicLoader::OpenLibrary(const kwsys_stl::string&
 //----------------------------------------------------------------------------
 int DynamicLoader::CloseLibrary(DynamicLoader::LibraryHandle lib)
 {
+  if (!lib)
+    {
+    return 0;
+    }
   return !shl_unload(lib);
 }