Fix string.char() recording with no arguments.

(cherry picked from commit dfa692b)

`string.char()` call without arguments yields an empty string. When JIT
machinery records the aforementioned call it doesn't handle case without
arguments. Each recording fast function expects 1 result by default.
Hence, when return from this call is recorded the framelink slot is used
as a result. 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 TV 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.

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 +1,28 @@
+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, 2 -- instruction becomes hot
+-- 3 -- trace is recorded (considering loop recording specifics),
+-- but bytecodes are still executed via VM
+-- 4 -- trace is executed, need to check that emitted mcode is
+--      correct
+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)