Two fixes for context#util#map_H

[jeremy-quicklearner]

Hello Christian! I've been using context.vim for over a year and it's been great :) Thanks very much for considering my PR.

First issue

When context#util#map_H is deciding which line to jump to, it accounts for the border of context.vim's popup window even when the popup window isn't present. So if there is no popup window, it jumps to the wrong line - just one line below where it should be going.

The fix is to only account for the border if w:context.lines is non-empty. If there is no w:context.lines, then there is no popup window and no border to account for.

Second issue

If I understand correctly, context#util#map_H emits an \<Esc> in order to clear the already-typed count... but if no count was passed, there is nothing for the \<Esc> to clear. On my terminal, this causes an annoying chirp sound.

The fix is to emit the \<Esc> only if the user typed a count.