Fix unexpected errors when decoding truncated QOI images

[abhi-0203]

Summary

Fixes a crash in QoiDecoder.decode() when a truncated QOI file is opened. The decoder now raises OSError with the standard "image file is truncated" message instead of an uncaught IndexError.

Problem

When a truncated QOI file is opened, QoiDecoder.decode() reads past EOF and indexes the empty result of read(1), causing an IndexError:

byte = self.fd.read(1)[0]  # IndexError: index out of range when read returns b''

This affects multiple read sites in the decoder:

• The initial opcode byte read (line 64)
• The QOI_OP_RGB 3-byte payload read (line 67)
• The QOI_OP_RGBA 4-byte payload read (line 69)
• The QOI_OP_LUMA second byte read (line 89)

Fix

Added length checks before indexing all read() results. When a short read is detected (EOF), the decoder raises OSError("image file is truncated"), consistent with other Pillow decoders (JPEG, PNG, etc.).

Testing

The fix can be verified by opening a truncated QOI file:

from PIL import Image
im = Image.open("truncated.qoi")
im.load()  # Previously: IndexError, Now: OSError("image file is truncated")

Fixes #9812

[radarhere]

I've created abhi-0203#1 with a suggestion.