sexp: Fix a memory leak

Fixes: CID 468917

Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>

diff -ur ../libgcrypt-1.11.0/src/sexp.c grub-core/lib/libgcrypt/src/sexp.c
--- a/grub-core/lib/libgcrypt-grub/src/sexp.c	2024-03-28 10:07:27.000000000 +0000
+++ b/grub-core/lib/libgcrypt-grub/src/sexp.c	2025-07-02 17:10:32.714864459 +0000
@@ -2725,8 +2725,10 @@
   length = 0;
   for (s=string; *s; s +=2 )
     {
-      if (!hexdigitp (s) || !hexdigitp (s+1))
+      if (!hexdigitp (s) || !hexdigitp (s+1)) {
+	free (buffer);
         return NULL;           /* Invalid hex digits. */
+      }
       ((unsigned char*)buffer)[length++] = xtoi_2 (s);
     }
   *r_length = length;