C_Digest(): check context validity before logging
C_Digest(): check context validity before logging

If the context is invalid (no C_Initialize) the we get a:
log.c:76: sc_do_log_va: Assertion `ctx != ((void *)0)' failed.

gemini/OpenSC,0x7678/OpenSC,LudovicRousseau/OpenSC,gentoo/OpenSC,kasparsd/opensc-latvia-id,mtrojnar/OpenSC,kasparsd/opensc-latvia-id,martinpaljak/OpenSC,dengert/OpenSC,marschap/pkg-opensc,ieugen/OpenSC,marschap/pkg-opensc,dirkx/OpenSC.tokend,l1k/OpenSC,gentoo/OpenSC,LudovicRousseau/OpenSC,martinpaljak/OpenSC,tidatida/OpenSC,martinpaljak/OpenSC,financeX/OpenSC,0x7678/OpenSC,aobaid/OpenSC,germanblanco/OpenSC,hongquan/OpenSC-main,metsma/OpenSC,hhonkanen/OpenSC,carlhoerberg/OpenSC,jpki/OpenSC,gentoo/OpenSC,aobaid/OpenSC,nmav/OpenSC,adminmt/OpenSC,metsma/OpenSC,dirkx/OpenSC.tokend,CardContact/OpenSC,marschap/pkg-opensc,philipWendland/OpenSC,jpki/OpenSC,velter/OpenSC,kasparsd/opensc-latvia-id,ieugen/OpenSC,0x7678/OpenSC,fabled/OpenSC,rickyepoderi/OpenSC,dirkx/OpenSC,carlhoerberg/OpenSC,CardContact/OpenSC,adminmt/OpenSC,marschap/pkg-opensc,Jakuje/OpenSC,carlhoerberg/OpenSC,OpenSC/OpenSC,Jakuje/OpenSC,UIKit0/OpenSC,fabled/OpenSC,adminmt/OpenSC,frankmorgner/OpenSC,viktorTarasov/OpenSC-SM,AktivCo/OpenSC,CardContact/OpenSC,jpki/OpenSC,adminmt/OpenSC,dirkx/OpenSC,viktorTarasov/OpenSC-SM,mouse07410/OpenSC,ieugen/OpenSC,0x7678/myOpenSC,rickyepoderi/OpenSC,AktivCo/OpenSC,philipWendland/OpenSC,hongquan/OpenSC-main,philipWendland/OpenSC,rickyepoderi/OpenSC,germanblanco/OpenSC,dirkx/OpenSC.tokend,mtrojnar/OpenSC,frankmorgner/OpenSC,Jakuje/OpenSC,frankmorgner/OpenSC,financeX/OpenSC,fabled/OpenSC,velter/OpenSC,UIKit0/OpenSC,hhonkanen/OpenSC,hongquan/OpenSC-main,nmav/OpenSC,kasparsd/opensc-latvia-id,0x7678/myOpenSC,aobaid/OpenSC,velter/OpenSC,dirkx/OpenSC.tokend,germanblanco/OpenSC,0x7678/myOpenSC,tidatida/OpenSC,l1k/OpenSC,viktorTarasov/OpenSC-SM,OpenSC/OpenSC,dirkx/OpenSC.tokend,tidatida/OpenSC,metsma/OpenSC,Jakuje/OpenSC,dengert/OpenSC,dirkx/OpenSC,dirkx/OpenSC,tidatida/OpenSC,ieugen/OpenSC,gemini/OpenSC,LudovicRousseau/OpenSC,frankmorgner/OpenSC,financeX/OpenSC,l1k/OpenSC,mouse07410/OpenSC,UIKit0/OpenSC,mouse07410/OpenSC,hhonkanen/OpenSC,nmav/OpenSC,carlhoerberg/OpenSC,dengert/OpenSC,gentoo/OpenSC,gemini/OpenSC,0x7678/OpenSC,0x7678/myOpenSC,OpenSC/OpenSC,ieugen/OpenSC,aobaid/OpenSC,0x7678/myOpenSC,fabled/OpenSC,mtrojnar/OpenSC,financeX/OpenSC,financeX/OpenSC,velter/OpenSC,carlhoerberg/OpenSC,dirkx/OpenSC.tokend,dirkx/OpenSC,AktivCo/OpenSC,UIKit0/OpenSC,gentoo/OpenSC