Prevent overflow and panic on large HTTP responses

[lpar]

The round2 function in http.go didn't check for overflow when converting a uint32 value +1 to an int. As a result, it could return 0 for large values, and hence attempting to handle a large response could result in a panic:

panic: runtime error: slice bounds out of range [:2930135446] with capacity 0

goroutine 36966 [running]:
github.com/valyala/fasthttp.appendBodyFixedSize(0xa1a1f8?, {0xa49b78?, 0x0?, 0x0?}, 0xa49b78?)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/http.go:2166 +0x214
github.com/valyala/fasthttp.readBody(0xa1a140?, 0xc0403869c0?, 0xc02ef77cd0?, {0xa49b78?, 0xc02ef77cd0?, 0x6918e5?})
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/http.go:2085 +0x5d
github.com/valyala/fasthttp.(*Response).ReadBody(0xc00060ab60, 0xc0403869c0?, 0x0?)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/http.go:1379 +0x186
github.com/valyala/fasthttp.(*Response).ReadLimitBody(0xc00060ab60, 0x7d7a98?, 0xc03ed69880?)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/http.go:1348 +0xd8
github.com/valyala/fasthttp.(*HostClient).doNonNilReqResp(0xc001c47d40, 0xc000628a80, 0xc00060ab60)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/client.go:1480 +0x570
github.com/valyala/fasthttp.(*HostClient).do(0xc0437abce0?, 0x0?, 0xc00060ab60)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/client.go:1372 +0x79
github.com/valyala/fasthttp.(*HostClient).Do(0xc001c47d40, 0xc000628a80, 0xc0437abce0?)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/client.go:1319 +0x91
github.com/valyala/fasthttp.(*Client).Do(0xc0000dc900, 0xc000628a80, 0x0?)
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/client.go:542 +0x59f
github.com/valyala/fasthttp.clientDoDeadline.func1()
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/client.go:1245 +0x79
created by github.com/valyala/fasthttp.clientDoDeadline
	/home/meta/go/pkg/mod/github.com/valyala/fasthttp@v1.38.0/client.go:1243 +0x46d

This patch fixes round2 to return a maximum value of math.MaxInt32, which seems to make sense given that the calculation is done using a uint32, and you probably don't want a bigger buffer than that even on 64 bit systems.

[erikdubbelboer]

Thanks!