Use correct length for the string returned from wxFile::ReadAll()

This corrects f75387850b and just uses the
newly added assign() overload taking the buffer directly: as the buffer
already knows its length, this makes it unnecessary to pass it
separately.

Closes #16490.

src/common/file.cpp

@@ -281,8 +281,7 @@ bool wxFile::ReadAll(wxString *str, const wxMBConv& conv)
         length -= nread;
     }
 
-    wxString strTmp(buf, conv, length);
-    str->swap(strTmp);
+    str->assign(buf, conv);
 
     return true;
 }