stream decompression: check 1~4 bytes Magic Number

When using stream decompression without ZSTD_f_zstd1_magicless:

• Feed 1~4 invalid bytes (wrong Magic Number), it doesn't report an error.
• Feed 5 invalid bytes, it reports "Unknown frame descriptor" as expected.

Magic Number is used to verify invalid data, so it could report an error earlier.
In some environments, short invalid data is common: urllib3/urllib3#2624 (comment)