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>.