[Fix] Check that merlin_home is present in autoload/merlin.vim

[leostera]

Hey guys! First of all thanks for Merlin. The badassery it brings to writing Ocaml in vim is unrivalled.

From a fresh start on macOS 10.12.5, with vim 8.0.1666, I got this from the get go:

Error detected while processing function merlin#Register[117]..merlin#LoadProject:
line    2:
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/ostera/.opam/system/share/merlin/vim/autoload/merlin.py", line 740, in setup_merlin
    failures = command("project","get")
  File "/Users/ostera/.opam/system/share/merlin/vim/autoload/merlin.py", line 201, in command
    return merlin_process().command(context(cmd))
  File "/Users/ostera/.opam/system/share/merlin/vim/autoload/merlin.py", line 175, in merlin_process
    instance = vim.eval("merlin#SelectBinary()")
vim.error: Vim(let):E716: Key not present in Dictionary: merlin_home."/".v:val
E171: Missing :endif
Error detected while processing function <SNR>25_LoadFTPlugin:
line   17:
E170: Missing :endfor

Apparently the code initially generated by opam user-setup install on my vimrc didn't set up the right globals? It seems the code is looking for s:c.merlin_home (https://github.com/ocaml/merlin/blob/master/vim/merlin/autoload/merlin.vim#L676) which in turn comes from g:merlin (https://github.com/ocaml/merlin/blob/master/vim/merlin/autoload/merlin.vim#L1), which wasn't set anywhere.

So after manually adding it to my already existing ftplugin/ocaml.vim, everything started to work again.

Here's my 2 cents, checking that s:c.merlin_home actually is set before trying to use it.

If there's anything else I need to do, or if you think there's a better way of handling this, I'm happy to hear out and put some more work in it 👍

[let-def]

Thanks, the fix is good.
This merlin_home feature should probably be removed or implemented in a better way, I will think about that, but that's beyond the scope of the PR.