patch 8.1.0935: old regexp engine may use invalid buffer

Problem:    Old regexp engine may use invalid buffer for 'iskeyword' or
            uninitialized buffer pointer. (Kuang-che Wu)
Solution:   Set rex.reg_buf when compiling the pattern. (closes #3972)

src/version.c

@@ -779,6 +779,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    935,
 /**/
     934,
 /**/