lib/alloc/realloc*.h: Always reallocate at least 1 byte

lib/alloc/realloc*.h: Always reallocate at least 1 byte

glibc's realloc(3) is broken.  It was originally good (I believe) until
at some point, when it was changed to conform to C89, which had a bogus
specification that required that it returns NULL.  C99 fixed the mistake
from C89, and so glibc's realloc(3) is non-conforming to
C99/C11/POSIX.1-2008.  C17 broke again the definition of realloc(3).

Link: <https://github.com/shadow-maint/shadow/pull/1095>
Link: <https://nabijaczleweli.xyz/content/blogn_t/017-malloc0.html>
Link: <https://inbox.sourceware.org/libc-alpha/5gclfbrxfd7446gtwd2x2gfuquy7ukjdbrndphyfmfszxlft76@wwjz7spd4vd7/T/#t>
Co-developed-by: наб <nabijaczleweli@nabijaczleweli.xyz>
Signed-off-by: наб <nabijaczleweli@nabijaczleweli.xyz>
Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Acked-by: Paul Eggert <eggert@cs.ucla.edu>
Signed-off-by: Alejandro Colomar <alx@kernel.org>