Fix #506 - Don't set signs for buffers which don't exist

autoload/ale/sign.vim

@@ -169,6 +169,12 @@ endfunction
 
 " This function will set the signs which show up on the left.
 function! ale#sign#SetSigns(buffer, loclist) abort
+    if !bufexists(str2nr(a:buffer))
+        " Stop immediately when attempting to set signs for a buffer which
+        " does not exist.
+        return
+    endif
+
     " Find the current markers
     let l:current_sign_list = ale#sign#FindCurrentSigns(a:buffer)
 

test/sign/test_sign_placement.vader

@@ -132,3 +132,6 @@ Execute(Loclist items with sign_id values should be kept):
   \   ['16', '1000004', 'ALEErrorSign'],
   \ ],
   \ ParseSigns()
+
+Execute(No excpetions should be thrown when setting signs for invalid buffers):
+  call ale#sign#SetSigns(123456789, [{'lnum': 15, 'col': 2, 'type': 'W', 'text': 'e'}])