libdwfl: fix OOB read in elf_from_remote_memory phdr check

libdwfl: fix OOB read in elf_from_remote_memory phdr check

phoff (e_phoff) comes straight from the untrusted file header, and the
sum is computed in unsigned 64-bit with no overflow check.

	* libdwfl/elf-from-memory.c (elf_from_remote_memory): Use an
	overflow-safe bound when checking whether the phdrs are already
	in the initial read.
	* tests/elf-from-memory.c: New file.
	* tests/Makefile.am (check_PROGRAMS): Add elf-from-memory.
	(TESTS): Likewise.
	(elf_from_memory_LDADD): New variable.

Signed-off-by: Matej Smycka <matejsmycka@gmail.com>