V850/BFD: Fix uninitialized `insn[4]' with R_V850_LONGJUMP warning

V850/BFD: Fix uninitialized `insn[4]' with R_V850_LONGJUMP warning

Correctly point at `insn[2]' rather than `insn[4]' in determining that
the third instruction from where an R_V850_LONGJUMP relocation points at
is not a recognized one, fixing a problem with commit 86aba9dbfa7e ("Add
linker relaxation to v850 toolchain") resulting in a build failure like:

.../bfd/elf32-v850.c: In function 'v850_elf_relax_section':
.../bfd/elf32-v850.c:3854:5: error: 'insn[4]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
 3854 |     _bfd_error_handler
      |     ^~~~~~~~~~~~~~~~~~
 3855 |       /* xgettext:c-format */
      |       ~~~~~~~~~~~~~~~~~~~~~~~
 3856 |       (_("%pB: %#" PRIx64 ": warning: %s points to "
      |       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 3857 |          "unrecognized insn %#x"),
      |          ~~~~~~~~~~~~~~~~~~~~~~~~~
 3858 |        abfd,
      |        ~~~~~
 3859 |        (uint64_t) (irel->r_offset + no_match),
      |        ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 3860 |        "R_V850_LONGJUMP",
      |        ~~~~~~~~~~~~~~~~~~
 3861 |        insn[no_match]);
      |        ~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
Makefile:1608: recipe for target 'elf32-v850.lo' failed
make[4]: *** [elf32-v850.lo] Error 1

with GCC 10.

	bfd/
	* elf32-v850.c (v850_elf_relax_section): Fix the index used for
	reporting an unrecognized instruction with R_V850_LONGJUMP.