libdw: Don't use a FDE if it doesn't actually cover a real code range.

libdw: Don't use a FDE if it doesn't actually cover a real code range.

If the FDE end <= start then it doesn't actually cover a valid code range.
Don't use and cache such FDEs (it will cause memory leaks if there is
another FDE that has the same start address and a valid code range).

Such FDEs have been seen in the backtrace.ppc.exec testfile.

Signed-off-by: Mark Wielaard <mjw@redhat.com>