tree-wide: make new/new0/malloc_multiply/reallocarray safe for size 0

All underlying glibc calls are free to return NULL if the size argument
is 0. We most often call those functions with a fixed argument, or at least
something which obviously cannot be zero, but it's too easy to forget.

E.g. coverity complains about "rows = new0(JsonVariant*, n_rows-1);" in
format-table.c There is an assert that n_rows > 0, so we could hit this
corner case here. Let's simplify callers and make those functions "safe".

CID #1397035.

The compiler is mostly able to optimize this away:
$ size build{,-opt}/src/shared/libsystemd-shared-239.so
(before)
   text	   data	    bss	    dec	    hex	filename
2643329	 580940	   3112	3227381	 313ef5	build/src/shared/libsystemd-shared-239.so     (-O0 -g)
2170013	 578588	   3089	2751690	 29fcca	build-opt/src/shared/libsystemd-shared-239.so (-03 -flto -g)
(after)
   text	   data	    bss	    dec	    hex	filename
2644017	 580940	   3112	3228069	 3141a5	build/src/shared/libsystemd-shared-239.so
2170765	 578588	   3057	2752410	 29ff9a	build-opt/src/shared/libsystemd-shared-239.so

src/basic/alloc-util.c

@@ -12,7 +12,7 @@ void* memdup(const void *p, size_t l) {
 
         assert(l == 0 || p);
 
-        ret = malloc(l);
+        ret = malloc(l ?: 1);
         if (!ret)
                 return NULL;