Fix crystal-lang non file-tied message handling

Some messages of the crystal compiler are not tied to a file.
This causes a 'Key not present in Dictionnary' error (E716).
For the record, the json output on ```require "./nonexistent.cr"```
is the following :
```json
  [
  { "file":"/tmp/file.cr", "line":1, "column":1, "size":0,
    "message":"while requiring \"./nonexistent.cr\"" },
  { "message":"can't find file './nonexistent.cr' relative to '/tmp'" }
  ]
```
The second message does not have line/column attributes.

ale_linters/crystal/crystal.vim

@@ -5,6 +5,10 @@ function! ale_linters#crystal#crystal#Handle(buffer, lines) abort
     let l:output = []
 
     for l:error in ale#util#FuzzyJSONDecode(a:lines, [])
+        if !has_key(l:error, 'file')
+            continue
+        endif
+
         call add(l:output, {
         \   'lnum': l:error.line + 0,
         \   'col': l:error.column + 0,