Revert CatalogItem::GetTranslation() optimization

Partially reverts 1ed2ac3.

Don't specialize GetTranslation() (i.e. n=0) to return a reference. It
doesn't have measurable effect after the other optimizations and it
causes crashes in code that implicitly assumes that GetTranslation() is
safe to call e.g. on non-translation files too (see e.g. 6b893be).

It's just not worth the risk.

src/catalog.cpp

@@ -867,7 +867,7 @@ void CatalogItem::SetFuzzy(bool fuzzy)
 wxString CatalogItem::GetTranslation(unsigned idx) const
 {
     if (idx >= GetNumberOfTranslations())
-        return wxEmptyString;
+        return wxString();
     else
         return m_translations[idx];
 }

src/catalog.h

@@ -144,8 +144,7 @@ class CatalogItem
         unsigned GetPluralFormsCount() const;
 
         /// Returns the nth-translation.
-        wxString GetTranslation(unsigned n) const;
-        const wxString& GetTranslation() const { return m_translations[0]; }
+        wxString GetTranslation(unsigned n = 0) const;
 
         /// Returns all translations.
         const wxArrayString& GetTranslations() const { return m_translations; }