lib/sprintf.[ch]: Add x[v]asprintf()

As other x...() wrappers around functions that allocate, these wrappers
are like [v]asprintf(3), but exit on failure.

Reviewed-by: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>

lib/sprintf.c

@@ -0,0 +1,19 @@
+/*
+ * SPDX-FileCopyrightText: 2023, Alejandro Colomar <alx@kernel.org>
+ * SPDX-License-Identifier: BSD-3-Clause
+ */
+
+#include <config.h>
+
+#ident "$Id$"
+
+#include "sprintf.h"
+
+#include <stdarg.h>
+#include <stddef.h>
+#include <stdio.h>
+
+
+extern inline int xasprintf(char **restrict s, const char *restrict fmt, ...);
+extern inline int xvasprintf(char **restrict s, const char *restrict fmt,
+    va_list ap);