Suppress -Wnull-dereference in wxString::GetCacheElement()

It doesn't seem to be really possible here.

include/wx/string.h

@@ -569,6 +569,12 @@ private:
   // its corresponding index in the byte string or not
   Cache::Element *GetCacheElement() const
   {
+      // gcc warns about cacheBegin and c inside the loop being possibly null,
+      // but this shouldn't actually be the case
+#if wxCHECK_GCC_VERSION(6,1)
+      wxGCC_ONLY_WARNING_SUPPRESS(null-dereference)
+#endif
+
       Cache::Element * const cacheBegin = GetCacheBegin();
       Cache::Element * const cacheEnd = GetCacheEnd();
       Cache::Element * const cacheStart = cacheBegin + LastUsedCacheElement();
@@ -598,6 +604,10 @@ private:
       }
 
       return c;
+
+#if wxCHECK_GCC_VERSION(6,1)
+      wxGCC_ONLY_WARNING_RESTORE(null-dereference)
+#endif
   }
 
   size_t DoPosToImpl(size_t pos) const