Fix predict_next() in parser (for real now).

Reported by Sergey Kaplun.

(cherry picked from commit f602f01)

Before the patch `predict_next()` uses the pc allocation limit
(`fs->bclim`) instead of the real limit of the defined bytecodes
(`fs->pc`). This leads to the use of undefined value and possible
crash. This patch fixes the check.

Sergey Kaplun:
* added the description and the test for the problem

Part of tarantool/tarantool#10709

src/lj_parse.c

@@ -2527,11 +2527,9 @@ static void parse_for_num(LexState *ls, GCstr *varname, BCLine line)
 */
 static int predict_next(LexState *ls, FuncState *fs, BCPos pc)
 {
-  BCIns ins;
+  BCIns ins = fs->bcbase[pc].ins;
   GCstr *name;
   cTValue *o;
-  if (pc >= fs->bclim) return 0;
-  ins = fs->bcbase[pc].ins;
   switch (bc_op(ins)) {
   case BC_MOV:
     if (bc_d(ins) >= fs->nactvar) return 0;
@@ -2580,7 +2578,7 @@ static void parse_for_iter(LexState *ls, GCstr *indexname)
   assign_adjust(ls, 3, expr_list(ls, &e), &e);
   /* The iterator needs another 3 [4] slots (func [pc] | state ctl). */
   bcreg_bump(fs, 3+LJ_FR2);
-  isnext = (nvars <= 5 && predict_next(ls, fs, exprpc));
+  isnext = (nvars <= 5 && fs->pc > exprpc && predict_next(ls, fs, exprpc));
   var_add(ls, 3);  /* Hidden control variables. */
   lex_check(ls, TK_do);
   loop = bcemit_AJ(fs, isnext ? BC_ISNEXT : BC_JMP, base, NO_JMP);

test/tarantool-tests/lj-1226-fix-predict-next.test.lua

@@ -0,0 +1,31 @@
+local tap = require('tap')
+local test = tap.test('lj-1226-fix-predict-next')
+
+test:plan(3)
+
+-- The resulting bytecode is the following:
+--
+-- 0001    KNIL     0   3
+-- 0002    JMP      4 => 0003
+-- 0003 => ITERC    4   2   3
+-- 0004    ITERL    4 => 0003
+--
+-- The parsing of the `for` iterator uses the incorrect check for
+-- `fs->bclim`, which allows the usage of an uninitialized value,
+-- so the test fails under Valgrind.
+local res_f = loadstring([[
+-- This local variable is necessary, because it emits `KPRI`
+-- bytecode, with which the next `KPRI` bytecode will be merged.
+local _
+for _ in nil do end
+]])
+
+test:ok(res_f, 'chunk loaded successfully')
+
+local res, err = pcall(res_f)
+
+-- Check consistency with PUC Rio Lua 5.1 behaviour.
+test:ok(not res, 'loaded function not executed')
+test:like(err, 'attempt to call a nil value', 'correct error message')
+
+test:done(true)