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.

bfd/ChangeLog

@@ -1,3 +1,9 @@
+2019-03-21  Jim Wilson  <jimw@sifive.com>
+
+	PR 24365
+	* elfnn-riscv.c (riscv_elf_relocate_section): For STT_SECTION check,
+	verify sym non-NULL before using.  Add identical check using h.
+
 2019-03-21  Sudakshina Das  <sudi.das@arm.com>
 
 	* elf-bfd.h (struct elf_backend_data): Add argument to

bfd/elfnn-riscv.c

@@ -2059,7 +2059,9 @@ riscv_elf_relocate_section (bfd *output_bfd,
 	     all relocs to update these addends.  This is also ambiguous, as
 	     we do allow offsets to be added to the target address, which are
 	     not to be used to find the auipc address.  */
-	  if ((ELF_ST_TYPE (sym->st_info) == STT_SECTION) && rel->r_addend)
+	  if (((sym != NULL && (ELF_ST_TYPE (sym->st_info) == STT_SECTION))
+	       || (h != NULL && h->type == STT_SECTION))
+	      && rel->r_addend)
 	    {
 	      r = bfd_reloc_dangerous;
 	      break;