zsh4.0.1-pre4 : backticks fail if SHLVL nonexistant

[Richard Curnow <Richard.Curnow@xxxxxx>]

I've downloaded and built zsh4.0.1-pre4.  I was finding that any attempt to use
backtick-substitution was resulting in a core dump.  I rebuilt with -g and
tracked it down to a null pointer 'pm' in setiparam, for an attempt to set
SHLVL from line 2316 of exec.c.  Unfortunately, I had SHLVL being unset by a
line in my .zshenv.  (In order to reduce the environment to a clean initial
state before adding new defintions, I unset everything except for a defined
list of variables at the start of .zshenv.  Previously, SHLVL wasn't one of
them.  It worked OK with zsh v3.1.5).

It looks like the backticks code needs to be made robust against SHLVL not
existing (i.e. it should be created if missing, or the code shouldn't care if
it is missing.)

Cheers
Richard

-- 
Richard Curnow---by day : SuperH Core Architecture at STMicroelectronics
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