Attempt to fix build without assertions no. 7.

src/Magnum/Text/AbstractFontConverter.cpp

@@ -101,6 +101,7 @@ Containers::Array<char> AbstractFontConverter::doExportFontToSingleData(Abstract
 {
     CORRADE_ASSERT(false,
         "Text::AbstractFontConverter::exportFontToSingleData(): feature advertised but not implemented", nullptr);
+    return nullptr;
 }
 
 bool AbstractFontConverter::exportFontToFile(AbstractFont& font, GlyphCache& cache, const std::string& filename, const std::string& characters) const {
@@ -157,6 +158,7 @@ Containers::Array<char> AbstractFontConverter::exportGlyphCacheToSingleData(Glyp
 Containers::Array<char> AbstractFontConverter::doExportGlyphCacheToSingleData(GlyphCache&) const {
     CORRADE_ASSERT(false,
         "Text::AbstractFontConverter::exportGlyphCacheToSingleData(): feature advertised but not implemented", nullptr);
+    return nullptr;
 }
 
 bool AbstractFontConverter::exportGlyphCacheToFile(GlyphCache& cache, const std::string& filename) const {
@@ -210,6 +212,7 @@ std::unique_ptr<GlyphCache> AbstractFontConverter::importGlyphCacheFromSingleDat
 std::unique_ptr<GlyphCache> AbstractFontConverter::doImportGlyphCacheFromSingleData(Containers::ArrayReference<const char>) const {
     CORRADE_ASSERT(false,
         "Text::AbstractFontConverter::importGlyphCacheFromSingleData(): feature advertised but not implemented", nullptr);
+    return nullptr;
 }
 
 std::unique_ptr<GlyphCache> AbstractFontConverter::importGlyphCacheFromFile(const std::string& filename) const {