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Fix predict_next() in parser.

Reported by Sergey Kaplun.

(cherry-picked from commit caf7cbc57)

Assume, we have the following Lua code:
| local _
| for _ in (nil):foo() do end

The first part of the bytecode emitted for it is the following:
| 0001    KNIL     0   1
| 0002    MOV      2   1
| 0003    TGETS    1   1   0  ; "foo"
| 0004    CALL     1   4   2

The `0001 KNIL` is a result of merging two `KPRI` instructions: one for
the local variable, one for the slot with `nil` object. During parsing in
`predict_next()` the second `MOV` bytecode is examined to set `pairs` or
`next` local variable. But, as far as it moves `nil` value, that isn't
an actual variable and has no the name, so this leads to the crash.

This patch adds a check to be sure that `RD` (see <src/lj_bc.h>) in the
`MOV` bytecode is an actual variable.

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

Part of tarantool/tarantool#8825

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