readelf: Handle NULL phdr in handle_dynamic_symtab

readelf: Handle NULL phdr in handle_dynamic_symtab

A corrupt ELF file can have broken program headers, in which case
gelf_getphdr returns NULL. This could crash handle_dynamic_symtab
while searching for the PT_DYNAMIC phdr. Fix this by checking whether
gelf_phdr returns NULL.

	  * src/readelf.c (handle_dynamic_symtab): Check whether
          gelf_getphdr returns NULL.

https://sourceware.org/bugzilla/show_bug.cgi?id=32655

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