Improve _accept length check

[radarhere]

This improves/adds length checks in _accept for several plugins.

Here are two examples of the errors this fixes.

>>> from PIL import FliImagePlugin
>>> FliImagePlugin._accept(b"test\x11\xaf")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/FliImagePlugin.py", line 35, in _accept
    and i16(prefix, 14) in [0, 3]  # flags
  File "PIL/_binary.py", line 37, in i16le
    return unpack_from("<H", c, o)[0]
struct.error: unpack_from requires a buffer of at least 16 bytes for unpacking 2 bytes at offset 14 (actual buffer size is 6)
>>> from PIL import GribStubImagePlugin
>>> GribStubImagePlugin._accept(b"GRIB")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "PIL/GribStubImagePlugin.py", line 36, in _accept
    return prefix.startswith(b"GRIB") and prefix[7] == 1
IndexError: index out of range