Fix segfault in bookmark tags.
Fix segfault in bookmark tags.

Ensure we pre-initialise loop-conditioned variables when building up the tag
name, and don't rely on them being assigned default values, which can cause
integer overflows.

ThomasAdam/vimprobable,DeadDork/vimprobable2,hut/vimprobable,DeadDork/vimprobable2,desyncr/vimprobable,hut/vimprobable,desyncr/vimprobable,fanglingsu/vimprobable,hut/vimprobable,fanglingsu/vimprobable,fanglingsu/vimprobable,ThomasAdam/vimprobable,desyncr/vimprobable,DeadDork/vimprobable2,ThomasAdam/vimprobable,desyncr/vimprobable,DeadDork/vimprobable2