MFC r265613, r267649: Handle ELF files with 65280 or more sections

  If e_shnum or e_shstrndx are at least SHN_LORESERVE (0xff00) then an
  escape value is used to indicate that the actual value is found in one
  of section 0's fields.

  Don't dump core when the ELF file has no section headers. The ELF
  core files created by gcore are among those.