imgui_freetype: Fix for rare case where FT_Get_Char_Index() succeed but FT_Load_Glyph() fails. (#618)

docs/CHANGELOG.txt

@@ -56,6 +56,7 @@ Other Changes:
   BeginMenu()/EndMenu() or BeginPopup/EndPopup(). (#3223, #1207) [@rokups]
 - Drag and Drop: Fixed unintended fallback "..." tooltip display during drag operation when
   drag source uses _SourceNoPreviewTooltip flags. (#3160) [@rokups]
+- Misc, Freetype: Fix for rare case where FT_Get_Char_Index() succeed but FT_Load_Glyph() fails.
 - CI: Added CI test to verify we're never accidentally dragging libstdc++ (on some compiler setups,
   static constructors for non-pod data seems to drag in libstdc++ due to thread-safety concerns).
   Fixed a static contructor which led to this dependency on some compiler setups (unclear which).

misc/freetype/imgui_freetype.cpp

@@ -13,6 +13,7 @@
 // - v0.60: (2019/01/10) re-factored to match big update in STB builder. fixed texture height waste. fixed redundant glyphs when merging. support for glyph padding.
 // - v0.61: (2019/01/15) added support for imgui allocators + added FreeType only override function SetAllocatorFunctions().
 // - v0.62: (2019/02/09) added RasterizerFlags::Monochrome flag to disable font anti-aliasing (combine with ::MonoHinting for best results!)
+// - v0.63: (2020/06/04) fix for rare case where FT_Get_Char_Index() succeed but FT_Load_Glyph() fails. 
 
 // Gamma Correct Blending:
 //  FreeType assumes blending in linear space rather than gamma space.
@@ -467,7 +468,6 @@ bool ImFontAtlasBuildWithFreeType(FT_Library ft_library, ImFontAtlas* atlas, uns
             ImFontBuildSrcGlyphFT& src_glyph = src_tmp.GlyphsList[glyph_i];
 
             const FT_Glyph_Metrics* metrics = src_tmp.Font.LoadGlyph(src_glyph.Codepoint);
-            IM_ASSERT(metrics != NULL);
             if (metrics == NULL)
                 continue;
 
@@ -559,6 +559,8 @@ bool ImFontAtlasBuildWithFreeType(FT_Library ft_library, ImFontAtlas* atlas, uns
             ImFontBuildSrcGlyphFT& src_glyph = src_tmp.GlyphsList[glyph_i];
             stbrp_rect& pack_rect = src_tmp.Rects[glyph_i];
             IM_ASSERT(pack_rect.was_packed);
+            if (pack_rect.w == 0 && pack_rect.h == 0)
+                continue;
 
             GlyphInfo& info = src_glyph.Info;
             IM_ASSERT(info.Width + padding <= pack_rect.w);