accessing keys crashes with core 1.26 on armv7

[r10s]

core 1.26 with rpgp 0.4.1 crashes on armv7 when a key is accessed, eg. in dc_get_securejoin_qr() (first finding at deltachat/deltachat-android#1259) but also when sending a message or trying to export a backup.
in case of dc_get_securejoin_qr(), the crash seems to happen somewhere inside Key::from_slice(), see log at deltachat/deltachat-android#1259 (comment), i did not collected details about other crashes yet.

in the android-emulator, things work out a this uses the x86 target - same for all the ci.

core 1.25 works fine, just tested that.