Fix gcc12 -fanalyzer warning

GCC do not understand that `algname` cannot be NULL. Add dummy default case to
cover all execution paths.

Fixes https://github.com/gost-engine/engine/issues/389


Tested-by: Ilya Shipitsin <chipitsine@gmail.com>
Signed-off-by: Vitaly Chikunov <vt@altlinux.org>