'main': Fix regression in zsh 5.3.1 and older: all precmd hooks later than z-sy-h would be aborted.

In those versions of zsh, «[[ -o nosuchoption ]]» is regarded as
a syntax error.  In newer zsh versions, it merely returns non-zero
(specifically, it returns 3, unlike «[[ -o unsetoption ]]» which
returns 1).

Fixes #732.
Fixes #733.

changelog.md

@@ -77,7 +77,7 @@
 
 - Fix an error message on stderr before every prompt when the `WARN_NESTED_VAR` zsh option is set:
   `_zsh_highlight_main__precmd_hook:1: array parameter _zsh_highlight_main__command_type_cache set in enclosing scope in function _zsh_highlight_main__precmd_hook`
-  [#727, #731]
+  [#727, #731, #732, #733]
 
 # Changes in version 0.7.1