Memory leaks and ineffective bounds checking in wasm_scan

Memory leaks and ineffective bounds checking in wasm_scan

It's always a bad idea to perform arithmetic on an unknown value read
from an object file before comparing against bounds.  Code like the
following attempting to bounds check "len", a 64-bit value, isn't
effective because the pointer arithmetic ignores the high 32 bits when
compiled for a 32-bit host.

      READ_LEB128 (len, p, end);
      if (p + len < p || p + len > end)
        goto error_return;

Instead, perform any arithmetic on known values where we don't need to
worry about overflows:

      READ_LEB128 (len, p, end);
      if (len > (size_t) (end - p))
        goto error_return;

I'll note that this check does do things the right way:

  READ_LEB128 (symcount, p, end);
  /* Sanity check: each symbol has at least two bytes.  */
  if (symcount > payload_size / 2)
    return FALSE;

"symcount * 2 > payload_size" would be wrong since the multiply could
overflow.

	* wasm-module.c (wasm_scan_name_function_section): Formatting.
	Delete asect name check.  Move asect NULL check to wasm_object_p.
	Correct bounds check of sizes against end.  Replace uses of
	bfd_zalloc with bfd_alloc, zeroing only necessary bytes.  Use
	just one bfd_release.
	(wasm_scan): Don't use malloc/strdup for section names,
	bfd_alloc instead.  Simplify code prefixing section name.
	Formatting.  Don't attempt to free memory here..
	(wasm_object_p): ..do so here.  Formatting.