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/regexp.c

@@ -7998,6 +7998,8 @@ vim_regcomp(char_u *expr_arg, int re_flags)
     bt_regengine.expr = expr;
     nfa_regengine.expr = expr;
 #endif
+    // reg_iswordc() uses rex.reg_buf
+    rex.reg_buf = curbuf;
 
     /*
      * First try the NFA engine, unless backtracking was requested.