[BUG] E15 error is called for valid expression inside a if with false condition

[ZyX]

Consider the following code:

    " test-filereadable.vim:
    set nocompatible
    let s:g={"load":{"saveoptions":{"abc":function("string")}}}
    function TestFileReadable()
        let file="/non-existent-file"
        let tail="abc"
        if filereadable(file)
            call s:g.load.saveoptions[tail](readfile(file, 'b'))
        endif
    endfunction
    try
        call TestFileReadable()
    catch
        call writefile([v:exception], "bad")
    endtry
    qa!

For fea3207b800d (vim-7.3.96) and earlier revisions this code does not throw 
any 
errors (because it contains none). For faf4b09c396e (vim-7.3.97) and later 
revisions we get the following contents in the «bad» file:

    Vim(call):E15: Invalid expression: 
s:g.load.saveoptions[tail](readfile(file, 
'b'))

(one line, of course). Expression s:g.load.saveoptions[tail](...) is actually 
used for existing files in one of my plugins and thus is valid.

Attaching files that were used to find revision where it was introduced with 
`hg 
bisect'. If you want to repeat bisecting, you will need to adjust paths in 
`test-bad.zsh'.

test-filereadable.vim
Description: application/wine-extension-vim

test-bad.zsh
Description: application/shellscript

signature.asc
Description: This is a digitally signed message part.