QIODevice makes readData() call only when its read buffer is empty.
Also data argument points to the user or reserved read buffer area.
So, no need in data transfer from read buffer at this point at all.

Task-number: QTBUG-41797
Change-Id: Ieb4afdf7eec37fdf288073e4a060e64424f22b9c
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
(cherry picked from commit 1853579dad1bbb44599314213a1d8a203ecae1c9)
Reviewed-by: Alex Trotsenko <alex1973tr@gmail.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>