src/signature: fix error return value in cms_decrypt()
src/signature: fix error return value in cms_decrypt()

cms_decrypt() returns a pointer, thus res should be set to NULL instead
of FALSE.
Also add a missing res = NULL for consistency reasons.

Signed-off-by: Enrico Jorns <a2e5f936c390f36494e46e71bcfbe365bb1e8c49@pengutronix.de>

jluebbe/rauc,jluebbe/rauc,rauc/rauc,ejoerns/rauc,ejoerns/rauc,jluebbe/rauc,rauc/rauc,rauc/rauc,ejoerns/rauc