Plug a memory leak when adding a font on some platforms

On macOS (with Core Text) and some versions of Windows (with DirectWrite)
(specifically, Windows 10+ Win32 as well as all versions of WinRT/UWP),
we read font metadata on our own via FreeType, filling implicit_meta in
ass_font_provider_add_font. However, we forgot to free it upon successful
font registration, leaking the memory allocated for the font name strings.

The list of affected platforms would likely have expanded in the future,
as we eventually intend to read font names on our own across the board.

Leak introduced in commit 887e6cc.

As a quick fix, add the corresponding frees on the successful path.
Ideally, we should refactor this function to avoid copying any freshly
allocated memory to begin with (and thus to avoid the need to free it).

Fixes: libass#699

libass/ass_fontselect.c

@@ -534,6 +534,10 @@ ass_font_provider_add_font(ASS_FontProvider *provider,
     info->provider = provider;
 
     selector->n_font++;
+
+    free_font_info(&implicit_meta);
+    free(implicit_meta.postscript_name);
+
     return true;
 
 error: