fix: adds validation for chunk sizes in unsigned streaming trailer upload

Fixes #1665

S3 enforces a validation rule for unsigned streaming payload trailer uploads: all chunk sizes must be greater than 8192 bytes except for the final chunk.
This fix adds a check in the unsigned chunk reader that validates chunk sizes by comparing each chunk size to the previous one.

s3err/s3err.go

@@ -180,6 +180,7 @@ const (
 	ErrInvalidLocationConstraint
 	ErrInvalidArgument
 	ErrMalformedTrailer
+	ErrInvalidChunkSize
 
 	// Non-AWS errors
 	ErrExistingObjectIsDirectory
@@ -804,6 +805,11 @@ var errorCodeResponse = map[ErrorCode]APIError{
 		Description:    "The request contained trailing data that was not well-formed or did not conform to our published schema.",
 		HTTPStatusCode: http.StatusBadRequest,
 	},
+	ErrInvalidChunkSize: {
+		Code:           "InvalidChunkSizeError",
+		Description:    "Only the last chunk is allowed to have a size less than 8192 bytes",
+		HTTPStatusCode: http.StatusBadRequest,
+	},
 
 	// non aws errors
 	ErrExistingObjectIsDirectory: {