[PATCH] avoid -Wuse-after-free [BZ #26779]

[Martin Sebor]

GCC 12 features a couple of new warnings designed to detect uses
of pointers made invalid by the pointees lifetimes having ended.
Building Glibc with the enhanced GCC exposes a few such uses,
mostly after successful calls to realloc.  The attached patch
avoids the new warnings by converting the pointers to uintptr_t
first and using the converted integers instead.

The patch suppresses all instances of the warning at the strictest
setting (-Wuse-after-free=3), which includes even uses in equality
expressions.  The default setting approved for GCC 12 is
-Wuse-after-free=2, which doesn't warn on such uses to accommodate
the pointer-adjustment-after-realloc idiom.  At the default setting,
the changes to ldconfig.c and setenv are not necessary.

Martin
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