support: Implement 'xfmemopen' for seamless 'fmemopen' use

Add 'xfmemopen' wrapper for seamless 'fmemopen' use in tests, following
'xfopen', 'xfclose', etc., and providing a standardized error reporting
facility.

Reviewed-by: Florian Weimer <fweimer@redhat.com>

support/Makefile

@@ -134,6 +134,7 @@ libsupport-routines = \
   xfclose \
   xfdopendir \
   xfgets \
+  xfmemopen \
   xfopen \
   xfork \
   xfread \

support/xfmemopen.c

@@ -0,0 +1,31 @@
+/* fmemopen with error checking.
+   Copyright (C) 2025 Free Software Foundation, Inc.
+   This file is part of the GNU C Library.
+
+   The GNU C Library is free software; you can redistribute it and/or
+   modify it under the terms of the GNU Lesser General Public
+   License as published by the Free Software Foundation; either
+   version 2.1 of the License, or (at your option) any later version.
+
+   The GNU C Library is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+   Lesser General Public License for more details.
+
+   You should have received a copy of the GNU Lesser General Public
+   License along with the GNU C Library; if not, see
+   <https://www.gnu.org/licenses/>.  */
+
+#include <support/xstdio.h>
+
+#include <support/check.h>
+#include <stdlib.h>
+
+FILE *
+xfmemopen (void *mem, size_t len, const char *mode)
+{
+  FILE *fp = fmemopen (mem, len, mode);
+  if (fp == NULL)
+    FAIL_EXIT1 ("fmemopen (mode \"%s\"): %m", mode);
+  return fp;
+}

support/xstdio.h

@@ -27,6 +27,7 @@ __BEGIN_DECLS
 FILE *xfopen (const char *path, const char *mode);
 void xfclose (FILE *);
 FILE *xfreopen (const char *path, const char *mode, FILE *stream);
+FILE *xfmemopen (void *mem, size_t len, const char *mode);
 void xfread (void *ptr, size_t size, size_t nmemb, FILE *stream);
 char *xfgets (char *s, int size, FILE *stream);