Bugfix FileStructure.struntil too short

Bug Description: FileStructure.struntil is off-by-one

Payload for stuff till 'v' where 'v' is a structure member. This payload includes 'v' as well.

For example, "flags" is the first 8 bytes of a FileStructructure:

>>> from pwn import *
>>> context.clear(arch="amd64")
>>> fp = FileStructure()
>>> payload = fp.struntil("flags")
>>> print(len(payload))
7

Expectation: len(payload) = 8

Actual Result: len(payload) = 7

Fix

The pull request fixes the issue by removing a [:-1] which truncated the last byte.