Fix panic if module by prefix is not found in Find()

Previously, if FindModuleByPrefix() returns nil in Find(), it would
continue to call module() with nil, resulting in a nil pointer
dereference panic when trying to call ParentNode() from RootNode().
The fix is to return an error if FindModuleByPrefix() returns nil.

Fixes #251

pkg/yang/entry.go

@@ -1322,11 +1322,17 @@ func (e *Entry) Find(name string) *Entry {
 			e = e.Parent
 		}
 		if prefix, _ := getPrefix(parts[0]); prefix != "" {
-			m := module(FindModuleByPrefix(contextNode, prefix))
-			if m == nil {
+			mod := FindModuleByPrefix(contextNode, prefix)
+			if mod == nil {
 				e.addError(fmt.Errorf("cannot find module giving prefix %q within context entry %q", prefix, e.Path()))
 				return nil
 			}
+			m := module(mod)
+			if m == nil {
+				e.addError(fmt.Errorf("cannot find which module %q belongs to within context entry %q",
+					mod.NName(), e.Path()))
+				return nil
+			}
 			if m != e.Node.(*Module) {
 				e = ToEntry(m)
 			}