Fix address violation when reading corrupt DWARF data.

Fix address violation when reading corrupt DWARF data.

	PR binutils/21648
	* dwarf.c (LEB): Rename to SKIP_ULEB and READ_ULEB.  Add check for
	reading a value that is too big for the containing variable.
	(SLEB): Rename to SKIP_SLEB and READ_SLEB.  Add similar check.
	Replace uses of LEB and SLEB with appropriate new macro.
	(display_debug_frames): Use an unsigned int for the 'reg'
	variable.  Use a signed long for the 'l' variable.