nimsuggest SIGSEGV when passing some unknown files

[timotheecour]

/cc @Araq now that https://github.com/nim-lang/Nim/pull/9757 was merged, the caveats I had mentioned in that PR hold:

nimsuggest --stdin /Users/timothee/git_clone/nim/Nim/lib/system.nim def /Users/timothee/git_clone/nim/Nim/lib/system.nim:106:22 def /Users/timothee/git_clone/nim/Nim/compiler/modules.nim:69:63 gives SIGSEGV see stacktrace here: https://github.com/nim-lang/Nim/pull/9715#issuecomment-438935200