bug#30564: call-process always fails for empty exec-path

[Christopher Wellons <wellons@nullprogram.com>]

The call-process function incorrectly fails for valid absolute paths 
when exec-path is nil:

    (let ((exec-path ()))
      (call-process "/bin/ls"))
    ;; error: (file-error "Searching for program"
    ;;                    "No such file or directory" "/bin/ls")

Adding a single element to exec-path fixes the problem, even if that 
element is nonsense:

    (let ((exec-path '(t)))
      (call-process "/bin/ls"))
    ;; => 0

The bug is in the for loop in openp() (lread.c). The only successful 
returns are found inside the loop. An empty list results in no loop 
iterations, which means an absolute path has no chance to return 
successfully.

This bug dates back at least as far as Emacs 24.3.