fmbt-editor: fix handling preview directives
fmbt-editor: fix handling preview directives

Fixes an issue when a directive (such as # preview-show-vars) is
the last line of the editor without line break.

heivi/fMBT,mixu-/fMBT,heivi/fMBT,acozzette/fMBT,01org/fMBT,pombreda/fMBT,mlq/fMBT,pombreda/fMBT,giantpinkwalrus/fMBT,01org/fMBT,heivi/fMBT,acozzette/fMBT,giantpinkwalrus/fMBT,pombreda/fMBT,mixu-/fMBT,mixu-/fMBT,yoonkiss/fMBT,giantpinkwalrus/fMBT,mlq/fMBT,mlq/fMBT,01org/fMBT,mixu-/fMBT,rseymour/fMBT,rseymour/fMBT,mlq/fMBT,giantpinkwalrus/fMBT,mixu-/fMBT,violep01/fMBT,pombreda/fMBT,pombreda/fMBT,yoonkiss/fMBT,rseymour/fMBT,pombreda/fMBT,violep01/fMBT,violep01/fMBT,rseymour/fMBT,01org/fMBT,rseymour/fMBT,yoonkiss/fMBT,violep01/fMBT,heivi/fMBT,giantpinkwalrus/fMBT,acozzette/fMBT,rseymour/fMBT,01org/fMBT,acozzette/fMBT,pombreda/fMBT,mlq/fMBT,yoonkiss/fMBT,rseymour/fMBT,heivi/fMBT,mlq/fMBT,mlq/fMBT,violep01/fMBT,heivi/fMBT,acozzette/fMBT,heivi/fMBT,yoonkiss/fMBT