Fix string.char() recording with no arguments.

(cherry picked from commit dfa692b)

`string.char()` call without arguments yields an empty string. JIT
recording machinery doesn’t handle this case. Each recording of a fast
function expects 1 result by default. Hence, when return from this call
is recorded some garbarge value is considered as a result to yield. It
is loaded into the corresponding slot as an IR with `IRT_NUM` type. It
leads to assertion failure in `rec_check_slots()`, when a next bytecode
is recorded, because type of TValue on the stack (`LJ_STR`) isn't the
same as IR (and TRef) type.

This patch handles the case without arguments by the loading of IR with
empty string reference into the corresponding slot. It reuses assumption
of one result by default, hence there is no case for `i == 1` in the
code.

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

Resolves tarantool/tarantool#6371

src/lj_ffrecord.c

@@ -866,6 +866,8 @@ static void LJ_FASTCALL recff_string_char(jit_State *J, RecordFFData *rd)
     for (i = 0; J->base[i] != 0; i++)
       tr = emitir(IRT(IR_BUFPUT, IRT_PGC), tr, J->base[i]);
     J->base[0] = emitir(IRT(IR_BUFSTR, IRT_STR), tr, hdr);
+  } else if (i == 0) {
+    J->base[0] = lj_ir_kstr(J, &J2G(J)->strempty);
   }
   UNUSED(rd);
 }

test/tarantool-tests/gh-6371-string-char-no-arg.test.lua

@@ -0,0 +1,29 @@
+local tap = require('tap')
+
+-- Test file to demonstrate assertion after `string.char()`
+-- recording.
+-- See also, https://github.com/tarantool/tarantool/issues/6371.
+
+local test = tap.test('gh-6371-string-char-no-arg')
+-- XXX: Number of loop iterations.
+-- * 1 -- instruction becomes hot.
+-- * 2 -- recording of the loop body.
+-- * 3 -- required for trace finalization, but this iteration runs the
+--   generated mcode. NB: the issue doesn't occur, since execution leaves
+--   the trace on table resizing guard.
+-- * 4 -- reproduces the issue.
+local NTEST = 4
+test:plan(NTEST)
+
+-- Storage for the results to avoid trace aborting by `test:ok()`.
+local results = {}
+jit.opt.start('hotloop=1')
+for _ = 1, NTEST do
+  table.insert(results, string.char())
+end
+
+for i = 1, NTEST do
+  test:ok(results[i] == '', 'correct recording of string.char() without args')
+end
+
+os.exit(test:check() and 0 or 1)