lib: fix maybe-uninitialized warnings

With -Og, GCC complains:

easy.c:628:7: error: ‘mcode’ may be used uninitialized in this function [-Werror=maybe-uninitialized]

../lib/strcase.h:35:29: error: ‘tok_buf’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
vauth/digest.c:208:9: note: ‘tok_buf’ was declared here

../lib/strcase.h:35:29: error: ‘tok_buf’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
vauth/digest.c:566:15: note: ‘tok_buf’ was declared here

Fix this by initializing the variables.