[Object] object::ELFObjectFile::symbol_begin(): skip symbol index 0

For clients iterating the symbol table, none expects to handle index 0
(STN_UNDEF). Skip it to improve consistency with other binary formats.
Clients that need STN_UNDEF (e.g. lld) can use
getSectionContentsAsArray(). A test will be added in D62148.

Reviewed By: mtrent

Differential Revision: https://reviews.llvm.org/D62296

llvm-svn: 361506