directory: converted isRootDirectory() to an inline function

The function isRootDirectory() is tiny and can be converted to an
inline function.  Don't allow name==NULL.

src/directory.c

@@ -101,11 +101,6 @@ void directory_finish(void)
 	freeDirectory(music_root);
 }
 
-int isRootDirectory(const char *name)
-{
-	return (!name || name[0] == '\0' || !strcmp(name, "/"));
-}
-
 struct directory *
 directory_get_root(void)
 {
@@ -122,6 +117,8 @@ getSubDirectory(struct directory * directory, const char *name)
 	char *duplicated;
 	char *locate;
 
+	assert(name != NULL);
+
 	if (isRootDirectory(name))
 		return directory;
 
@@ -148,6 +145,9 @@ getSubDirectory(struct directory * directory, const char *name)
 struct directory *
 getDirectory(const char *name)
 {
+	if (name == NULL)
+		return music_root;
+
 	return getSubDirectory(music_root, name);
 }
 

src/directory.h

@@ -46,7 +46,11 @@ void directory_init(void);
 
 void directory_finish(void);
 
-int isRootDirectory(const char *name);
+static inline bool
+isRootDirectory(const char *name)
+{
+	return name[0] == 0 || (name[0] == '/' && name[1] == 0);
+}
 
 struct directory *
 directory_get_root(void);