msan: use of uninitialised data in get_cie_info

msan: use of uninitialised data in get_cie_info

This completely bogus oss-fuzz x86 testcase results in a read from an
uninitialised (at the time check_eh_frame is called) part of an insn
frag:
 .section .debug_frame
 orl $1,x
 .long x
 .uleb128 0,x,0
x:

Fix the problem by verifying the assumption in get_cie_info that a CIE
starts at the beginning of .eh_frame or .debug_frame.  Or at least
exclude silliness involving instructions placed there.  That seems a
useful sanity check.  Also sanity check sizes of initial FDE fields.

Yes, this doesn't completely stop the problem since you could place an
insn with a relocated field later in the CIE.  If fuzzers find such a
testcase I'll ignore it.

	* ehopt.c (struct cie_info): Add "f" field.
	(get_cie_info): Return a bool.  Verify frag at start of chain
	is one with the CIE size found by check_eh_frame.
	(check_eh_frame): Save CIE start frag.  Only accept 4 or 8
	byte fields in state_saw_size, state_saw_cie_offset and
	state_saw_pc_begin.  Formatting.  Localise "fix" variable.