Fix []byte reuse bug

The return value of normalizePath should be a reusable []byte.
Returning strSlash could cause the strSlash constant to be reused and
resulted in very weird behavior. This specific case of normalizePath
almost never happens which made it even harder to spot.

The race detector on Windows was the only clue that something was
happening:

WARNING: DATA RACE
Write at 0x000001d082e3 by goroutine 446:
  github.com/valyala/fasthttp.addLeadingSlash()
      D:/a/fasthttp/fasthttp/uri_windows.go:8 +0xa5
  github.com/valyala/fasthttp.normalizePath()
      D:/a/fasthttp/fasthttp/uri.go:343 +0xbd
  github.com/valyala/fasthttp.(*URI).parse()
      D:/a/fasthttp/fasthttp/uri.go:314 +0x1606
  github.com/valyala/fasthttp.(*URI).Parse()
      D:/a/fasthttp/fasthttp/uri.go:266 +0xb3
  github.com/valyala/fasthttp.testURIPathNormalize()
      D:/a/fasthttp/fasthttp/uri_test.go:195 +0x47
  github.com/valyala/fasthttp.TestURIPathNormalize()
      D:/a/fasthttp/fasthttp/uri_test.go:176 +0x38e
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.15.15/x64/src/testing/testing.go:1123 +0x202

Previous read at 0x000001d082e3 by goroutine 332:
  runtime.slicecopy()
      C:/hostedtoolcache/windows/go/1.15.15/x64/src/runtime/slice.go:246 +0x0
  github.com/valyala/fasthttp.(*RequestHeader).AppendBytes()
      D:/a/fasthttp/fasthttp/header.go:1726 +0x24d
  github.com/valyala/fasthttp.(*RequestHeader).Header()
      D:/a/fasthttp/fasthttp/header.go:1698 +0x84
  github.com/valyala/fasthttp.(*RequestHeader).Write()
      D:/a/fasthttp/fasthttp/header.go:1682 +0x10f
  github.com/valyala/fasthttp.(*Request).Write()
      D:/a/fasthttp/fasthttp/http.go:1370 +0x1ab
  github.com/valyala/fasthttp.testRequestSuccess()
      D:/a/fasthttp/fasthttp/http_test.go:1788 +0x32a
  github.com/valyala/fasthttp.TestRequestSuccess()
      D:/a/fasthttp/fasthttp/http_test.go:1678 +0x2dd
  testing.tRunner()
      C:/hostedtoolcache/windows/go/1.15.15/x64/src/testing/testing.go:1123 +0x202

uri.go

@@ -390,7 +390,7 @@ func normalizePath(dst, src []byte) []byte {
 	if n >= 0 && n+len(strSlashDotDot) == len(b) {
 		nn := bytes.LastIndexByte(b[:n], '/')
 		if nn < 0 {
-			return strSlash
+			return append(dst[:0], strSlash...)
 		}
 		b = b[:nn+1]
 	}