pktools/lib/gba.py: added validations to GbaRom constructor
pktools/lib/gba.py: added validations to GbaRom constructor

Added extension and size validations to GbaRom constructor.

Signed-off-by: MegaMetapod <48ee6722255b619da9249aa56ff43c3552d60ba3@users.noreply.github.com>

MegaMetapod/pypktools