Detect invalid length field in debug frame FDE header.

Detect invalid length field in debug frame FDE header.

GDB was failing to catch cases where a corrupt ELF or core file
contained an invalid length value in a Dwarf debug frame FDE header.
It was checking for buffer overflow but not cases where the length was
negative or caused pointer wrap-around.

In addition to the additional validity check, this patch cleans up the
multiple signed/unsigned conversions on the length field so that an
unsigned representation is used consistently throughout.

This patch fixes CVE-2017-9778 and PR gdb/21600.

2019-04-25  Sandra Loosemore  <sandra@codesourcery.com>
	    Kang Li <kanglictf@gmail.com>

	PR gdb/21600

	* dwarf2-frame.c (read_initial_length): Be consistent about using
	unsigned representation of length.
	(decode_frame_entry_1): Likewise.  Check for wraparound of
	end pointer as well as buffer overflow.