PR28264, ld.bfd crash on linking efivar with LTO

	PR 28264
	PR 26978
	* linker.c (_bfd_generic_link_add_one_symbol <MIND>): Check
	that string is non-NULL.

bfd/linker.c

diff --git a/bfd/linker.c b/bfd/linker.c
index 755ff19..f8257ea 100644
--- a/bfd/linker.c
+++ b/bfd/linker.c
@@ -1682,7 +1682,7 @@
 	      cycle = true;
 	      break;
 	    }
-	  if (strcmp (h->u.i.link->root.string, string) == 0)
+	  if (string != NULL && strcmp (h->u.i.link->root.string, string) == 0)
 	    break;
 	  /* Fall through.  */
 	case MDEF: