bugfix: creating wav file on first run

We need to ensure the save dir exists. But it only gets created in calls
to love.filesystem.

wav.lua

@@ -23,8 +23,11 @@ wav = {
     if type(filename) ~= "string" then
       error("invalid function parameter, expected string filename", 2)
     end
+    local absolute_path = love.filesystem.getSaveDirectory()..'/'..filename
+    -- ensure directory exists
+    love.filesystem.write(absolute_path, '')
     -- Audio file handle
-    local file = io.open(love.filesystem.getSaveDirectory()..'/'..filename, "wb")
+    local file = io.open(absolute_path, "wb")
     if not file then
       error(string.format("couldn't open file %q", filename), 2)
     end