RISC-V: Fix linker crash in section symbol check.

sym is only set for local symbols.  h is only set for global symbols.  Gas
won't let me create a global section symbol, but bfd appears to have some
support for that, and I can't rule out that other assemblers might do this.
So we need to support both, and verify sym and h are non-NULL before using.

	bfd/
	PR 24365
	* elfnn-riscv.c (riscv_elf_relocate_section): For STT_SECTION check,
	verify sym non-NULL before using.  Add identical check using h.