libelf: elf_update should make sure section zero exists

libelf: elf_update should make sure section zero exists

elf_update might crash for 65280+ sections when section zero headers
aren't loaded. Make sure section zero is loaded before trying to write
to it.

Add testcase to tests/test-manyfuncs.c but use ELF_C_READ_MMAP_PRIVATE
to workaround https://sourceware.org/bugzilla/show_bug.cgi?id=33968

	* libelf/elf32_updatenull.c (updatenull_wrlock): Call
	getshdr_wrlock to load shdr.
	* tests/test-manyfuncs.c (check_elf): Use elf_begin with
        ELF_C_READ_MMAP_PRIVATE if use_mmap. Call elf_update early.

https://sourceware.org/bugzilla/show_bug.cgi?id=33967

Signed-off-by: Mark Wielaard <mark@klomp.org>