[committed] avoid invalid redeclarations (PR 97861)

[Martin Sebor]

When checking for mismatches between the array forms of arguments
between the current and the new declaration of a function
warn_parm_array_mismatch() assumes that the new declaration is
valid and compatible with the current one.  When that's not so
the function crashes with a null pointer dereference.  In r11-5213
I have committed the attached fix to avoid this unsafe assumption.

Martin

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