Pacify GCC -Wanalyzer-possible-null-dereference

This fixes the only remaining GCC diagnostics when emacs-28 is
configured with --enable-gcc-warnings.  It does so by adding
ATTRIBUTE_RETURNS_NONNULL so that GCC knows certain functions
return nonnull.  It also arranges for three of those functions to
always return nonnull; I thought these functions already were
doing so, but apparently not, and it is conceivable (though I
haven’t checked this) that changing these functions to always
return nonnull even on non-GNU platforms may fix unlikely
portability bugs elsewhere in Emacs.  I used GCC 11.2.1 20210728
(Red Hat 11.2.1-1) on x86-64 when checking the diagnostics.
* configure.ac: Invoke gl_EEMALLOC before gl_INIT, in case
the regex code doesn't invoke gl_EEMALLOC; needed for src/alloc.c’s
use of MALLOC_0_IS_NONNULL.
* src/alloc.c (xmalloc, xzalloc, xrealloc): Don’t worry about the
special case where SIZE == 0, since lmalloc and lrealloc now
return null only on allocation failure.
(lmalloc, lrealloc): Return null only on allocation failure,
instead of having special cases that treat malloc (0) and
realloc (X, 0) as successes even when they return null.
* src/lisp.h: Add ATTRIBUTE_RETURNS_NONNULL to a few functions
that always return nonnull pointers, so that gcc -fanalyzer
does not issue diagnostics like “alloc.c: In function
‘allocate_vector_block’: alloc.c:2985:15: warning: dereference of
possibly-NULL ‘block’ [CWE-690] [-Wanalyzer-possible-null-dereference]”
as per <https://cwe.mitre.org/data/definitions/690.html>.

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