wxOSX: don't call strlen() on possibly null pointer.

Use wxCharBuffer::length() instead, this is more efficient and safer as it
correctly returns 0 in case the conversion fails.

Closes #9967.

src/osx/utils_osx.cpp

@@ -274,7 +274,7 @@ CGColorRef wxMacCreateCGColorFromHITheme( ThemeBrush brush )
 void wxMacStringToPascal( const wxString&from , unsigned char * to )
 {
     wxCharBuffer buf = from.mb_str( wxConvLocal );
-    int len = strlen(buf);
+    size_t len = buf.length();
 
     if ( len > 255 )
         len = 255;