patch 9.1.0703: crash with 2byte encoding and glob2regpat()

Problem:  possible crash with 2-byte encoding and glob2regpat()
          (after v9.1.0700, v9.1.0702)
Solution: include both bytes for a multi-byte character for an
          escaped character

closes: #15590

Signed-off-by: Christian Brabandt <cb@256bit.org>

src/fileio.c

@@ -5714,6 +5714,8 @@ file_pat_to_reg_pat(
 				)
 			    *allow_dirs = TRUE;
 			reg_pat[i++] = '\\';
+			if (enc_dbcs != 0 && (*mb_ptr2len)(p) > 1)
+			    reg_pat[i++] = *p++;
 			reg_pat[i++] = *p;
 		    }
 		break;