Rcc: ensure enough space for the embedded

The two-pass feature depends on the QRC_DATA signature that needs
to be stored in the array later overwritten by the embedded data.

Task-number: QTBUG-42359
Change-Id: Ida1ccff40dda28f92a4267f86f48ee96d62bd214
Reviewed-by: aavit <eirik.aavitsland@theqtcompany.com>