libdw: Recognize zero terminator to end frame table in dwarf_next_cfi.

libdw: Recognize zero terminator to end frame table in dwarf_next_cfi.

When the length is zero this is a the zero terminator that ends the
frame table. Return 1 (end of table) instead of -1 (error) in that case.
We cannot update next_off and don't want to caller to try again.

Add testcase for dwarf_next_cfi to show both .eh_frame and .debug_frame
tables and check consistency (FDEs should point to existing CIEs).
Also add a self check to make sure we can read the table from the just
build elfutils binaries.

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