symbols_map: don't error on bad elf files

There are cc_binary_prebuilt modules in the tree that are shell
scripts, and attempting to extract an elf ID from them results
in an error.  Ignore too short files and files missing the elf
magic header the same way we do for elf files without an elf ID.

Bug: 218888599
Test: Test_elfIdentifierFromReaderAt_BadElfFile
Change-Id: If7117925ca2371a8ee61ba3616372f6e9b0fab0e