Do not run backticks in a file name from a tags file

expand() globs and runs backticks, so an entry naming '`touch FILE`.c' ran the
command, and on CTRL-O it ran even for an address the new check then refused.
fnamemodify(':p') instead, as s:in_dir already does for the same reason. It
gives up '$VAR' in a name, which no tag generator writes.

The address is now checked before the name is touched, so nothing happens at all
for an entry that will not be used.

autoload/fzf/vim.vim

@@ -772,13 +772,15 @@ function! s:goto_entry(winid, bufnr, dir, kind, entry) abort
       return
     endif
     let excmd = s:tag_address(join(parts[2:-2], '')[:-2], 1)
-    let relpath = parts[1][:-2]
-    let path = relpath =~ (s:is_win ? '^[A-Z]:\' : '^/')
-          \ ? relpath : join([fnamemodify(parts[-1], ':h'), relpath], '/')
-    let path = expand(path, 1)
     if empty(excmd)
       return s:warn('Unsupported tag address: '.s:strip(parts[0]))
     endif
+    let relpath = parts[1][:-2]
+    let path = relpath =~ (s:is_win ? '^[A-Z]:\' : '^/')
+          \ ? relpath : join([fnamemodify(parts[-1], ':h'), relpath], '/')
+    " fnamemodify(), not expand(), which runs backticks in the name a tags file
+    " gives, as s:in_dir says
+    let path = fnamemodify(path, ':p')
     if !filereadable(path)
       return
     endif
@@ -1800,10 +1802,12 @@ function! s:tags_sink(from, lines)
         let relpath = parts[1][:-2]
         let abspath = relpath =~ (s:is_win ? '^[A-Z]:\' : '^/') ? relpath : join([base, relpath], '/')
 
+        " fnamemodify(), not expand(), which runs backticks in the name
+        let abspath = fnamemodify(abspath, ':p')
         if len(list) == 1
-          call s:action_for(key, expand(abspath, 1))
+          call s:action_for(key, abspath)
         else
-          call s:open(expand(abspath, 1))
+          call s:open(abspath)
         endif
         call s:execute_tag_address(excmd)
         call add(qfl, {'filename': expand('%'), 'lnum': line('.'), 'text': getline('.')})