Fix ElfFile crashing when opening short (<64 bytes) files.
Fix ElfFile crashing when opening short (<64 bytes) files.

Summary:
ElfFile in init() assumes file is long enough to contain complete
ElfHeader, which is 64 bytes long. This is not true for valid non-ELF files like
short scripts. They shouldn't cause crash.

Reviewed By: myreg

Differential Revision: D6410210

fbshipit-source-id: 28fd017d8de17c431d7d006a1655ade8a95994bd

rklabs/folly,rklabs/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly,rklabs/folly,rklabs/folly,facebook/folly,rklabs/folly