Fix uninitialised variable errors

Fix uninitialised variable errors

Commit c6291d749aec introduced a number of errors, found by clang.

elf.c:456:7: error: variable 'alloc_ext_size' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
  if (_bfd_mul_overflow (symcount, extsym_size, &amt))
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
elf.c:464:7: error: variable 'alloc_extshndx_size' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
  if (bfd_seek (ibfd, pos, SEEK_SET) != 0
      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
elflink.c:2837:11: error: variable 'alloc1_size' is used uninitialized whenever 'if' condition is true [-Werror,-Wsometimes-uninitialized]
      if (internal_relocs == NULL)
          ^~~~~~~~~~~~~~~~~~~~~~~
elflink.c:12595:16: error: variable 'ext_size' set but not used [-Werror,-Wunused-but-set-variable]
                      size_t ext_size = 0;

	* elf.c (bfd_elf_get_elf_syms): Fix use of uninitialised variables.
	* elflink.c (_bfd_elf_link_info_read_relocs): Likewise.
	(bfd_elf_final_link): Fix set but not used warning.