L4TLauncher: buffer overrun / crash in ReadAndroidStyleKernelPartition with encrypted images and 2KB signatures

Mostly caused by e6fd382 "fix: verify signature with alignment 4KB and 2KB" (first included in L4T R36.5.0) but originally sourced to 21ff46f "L4T Launcher: enable UEFI to load encrypted payloads from partition". These are the buggy lines:
https://github.com/NVIDIA/edk2-nvidia/blob/fd77facdc6252a394f66b9c8dbd35d7f0166fa73/Silicon/NVIDIA/Application/L4TLauncher/L4TLauncher.c#L2057-L2059

1. SignatureOffset is aligned up to 4KB, which may be beyond DecryptedImageBufferSize when 2KB alignment was used to produce the image
2. SignatureSize underflows and becomes extremely large
3. CopyMem crashes due to a very large length parameter

There are also other issues here:

1. ImageBufferSize should ideally be checked to ensure it is not larger than DecryptedImageBufferSize before any of this code runs. That could cause a similar buffer overrun just with a corrupted image.
2. If there is even more than 4KB of padding at the end of the image, SignatureSize can become larger than the actual size of SignatureBuffer and cause a heap overrun.

Basically, SignatureSize should be clamped between 0 and the previous value of SignatureSize (4KB) on this line to resolve all issues.

The same issue exists in ReadAndroidStyleDtbPartition too, where SignatureSize can underflow:
https://github.com/NVIDIA/edk2-nvidia/blob/fd77facdc6252a394f66b9c8dbd35d7f0166fa73/Silicon/NVIDIA/Application/L4TLauncher/L4TLauncher.c#L2314

Start in Silicon/NVIDIA/Application/L4TLauncher/L4TLauncher.c at ReadAndroidStyleKernelPartition around lines 2057-2059, then inspect ReadAndroidStyleDtbPartition around line 2314. Exercise encrypted images using 2KB signatures, plus corrupted images and images with extra padding; done means SignatureSize cannot underflow or exceed its buffer and neither path overruns or crashes.