Handle zero size for posix_memalign. Return NULL or unique address

according to the 'V' option.

PR:	standards/138307
MFC after:	1 week

lib/libc/stdlib/malloc.c

@@ -5320,6 +5320,15 @@ posix_memalign(void **memptr, size_t alignment, size_t size)
 			goto RETURN;
 		}
 
+		if (size == 0) {
+			if (opt_sysv == false)
+				size = 1;
+			else {
+				result = NULL;
+				ret = 0;
+				goto RETURN;
+			}
+		}
 		result = ipalloc(alignment, size);
 	}