libelf: Add n_namesz offset overflow check to gelf_get_note.

libelf: Add n_namesz offset overflow check to gelf_get_note.

During fuzzing of the new xlate_notes testcase I noticed that
gelf_get_note didn't check whether the n_namesz of a note was
too big. This could lead to offset wrapping around. Causing an
infinite loop going over all ELF notes. Fix by adding an overflow
check before updating offset.

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