Secret finding is not detected when Read operation returns both n > 0 bytes and EOF

[dbrisson21]

Describe the bug
I have observed a scenario where a secret string is not properly detected if the Read() operation returns n > 0 bytes and the EOF flag.

Additional information:
According to the golang v1.22.2 documentation:

a Reader returning a non-zero number of bytes at the end of the input stream may return either err == EOF or err == nil.

The documentation also states that:
Callers should always process the n > 0 bytes returned before considering the error err.

To Reproduce
I have implemented the following unit test to help reproducing the issue :

package detect

import (
	"fmt"
	"io"
	"strings"
	"testing"

	"github.com/stretchr/testify/assert"
)

type mockRead func(secret []byte) (int, error)

func (m mockRead) Read(secret []byte) (int, error) {
	return m(secret)
}

// TestDetectReader tests the DetectReader function.
func TestDetectReader(t *testing.T) {
	var detector *Detector
	var secret = "AKIAIRYLJVKMPEXAMPLE"

	testString := strings.NewReader(secret)
	tests := []struct {
		name          string
		reader        io.Reader
		bufSize       int
		findingsCount int
	}{
		{
			name:          "Test case - Reader returns n > 0 bytes and nil error",
			bufSize:       10,
			findingsCount: 1,
			reader:        testString,
		},
		{
			name:          "Test case - Reader returns n > 0 bytes and io.EOF error", // this test case is failing with the current implementation
			bufSize:       10,
			findingsCount: 1,
			reader: mockRead(func(secret []byte) (int, error) {
				return 20, io.EOF
			}),
		},
	}

	for _, test := range tests {
		fmt.Printf("Running test case: %v\n", test.name)
		detector, _ = NewDetectorDefaultConfig()
		findings, _ := detector.DetectReader(test.reader, test.bufSize)
		assert.Equal(t, test.findingsCount, len(findings))
	}
}

Expected behavior
I am expecting the secret to be detected in both of these scenarios:

• n > 0 bytes returned, err == nil
• n > 0 bytes returned, err == io.EOF

Screenshots
N/A

Basic Info (please complete the following information):

• OS: Sonoma 14.3.1 Darwin Kernel Version 23.3.0
• Gitleaks Version: v8.18.2

Additional context

cc @zricethezav