fixes errors compiling netfs and netfs_server with gcc-4.7.3

Signed-off-by: Jérôme Duval <[email protected]>

src/add-ons/kernel/file_systems/netfs/client/ShareAttrDir.cpp

@@ -395,7 +395,7 @@ ShareAttrDir::GetAttribute(const char* name) const
 			return attribute;
 	}
 
-	return false;
+	return NULL;
 }
 
 // GetFirstAttribute

src/add-ons/kernel/file_systems/netfs/server/AttributeDirectory.cpp

@@ -329,7 +329,7 @@ AttributeDirectory::GetAttribute(const char* name) const
 			return attribute;
 	}
 
-	return false;
+	return NULL;
 }
 
 // GetFirstAttribute