gnulib: Bring back the fix for resolving unused variable issue

This patch resolved a minor issue spotted by Coverity:
  a983d36bd917 (gnulib/regexec: Resolve unused variable)

But, it was removed by the Gnulib update:
  2b7902459803 (Update gnulib version and drop most gnulib patches)

It caused Coverity to continue to flag the issue. Daniel Kiper
suggested to bring back the patch a983d36bd917 (gnulib/regexec: Resolve
unused variable).

Fixes: CID 292459

Signed-off-by: Lidong Chen <lidong.chen@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

bootstrap.conf

@@ -87,7 +87,8 @@ bootstrap_post_import_hook () {
   for patchname in fix-width \
       fix-regcomp-resource-leak \
       fix-regexec-resource-leak \
-      fix-gcc-15-compile; do
+      fix-gcc-15-compile \
+      fix-unused-value; do
     patch -d grub-core/lib/gnulib -p2 \
        < "grub-core/lib/gnulib-patches/$patchname.patch"
   done

conf/Makefile.extra-dist

@@ -32,6 +32,7 @@ EXTRA_DIST += grub-core/lib/gnulib-patches/fix-width.patch
 EXTRA_DIST += grub-core/lib/gnulib-patches/fix-regcomp-resource-leak.patch
 EXTRA_DIST += grub-core/lib/gnulib-patches/fix-regexec-resource-leak.patch
 EXTRA_DIST += grub-core/lib/gnulib-patches/fix-gcc-15-compile.patch
+EXTRA_DIST += grub-core/lib/gnulib-patches/fix-unused-value.patch
 
 EXTRA_DIST += grub-core/lib/libgcrypt
 EXTRA_DIST += grub-core/lib/libgcrypt-grub/mpi/generic

grub-core/lib/gnulib-patches/fix-unused-value.patch

@@ -0,0 +1,14 @@
+--- a/lib/regexec.c	2020-10-21 14:25:35.310195912 +0000
++++ b/lib/regexec.c	2020-10-21 14:32:07.961765604 +0000
+@@ -828,7 +828,11 @@
+ 		    break;
+ 		  if (__glibc_unlikely (err != REG_NOMATCH))
+ 		    goto free_return;
++#ifdef DEBUG
++		  /* Only used for assertion below when DEBUG is set, otherwise
++		     it will be over-written when we loop around.  */
+ 		  match_last = -1;
++#endif
+ 		}
+ 	      else
+ 		break; /* We found a match.  */