http2: remove suspicious uint32->v conversion in frame code

Function maxHeaderStringLen(...) uses uint32(int(v)) == v check to validate
if length will fit in the int type.
This check is a no-op on any architecture because int type always has at
least 32 bits, so we can potentially encounter negative return values from
maxHeaderStringLen(...) function. This can be bad as this outcome clearly
breaks code intention and maybe some further code invariants.

This patch replaces uint32(int(v)) == v check with more robust and simpler
int(v) > 0 validation which is correct for our case when we operating with
uint32

Fixes golang/go#64961

Change-Id: I31f95709df9d25593ade3200696ac5cef9f88652
Reviewed-on: https://go-review.googlesource.com/c/net/+/554235
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

http2/frame.go

@@ -1510,13 +1510,12 @@ func (mh *MetaHeadersFrame) checkPseudos() error {
 }
 
 func (fr *Framer) maxHeaderStringLen() int {
-	v := fr.maxHeaderListSize()
-	if uint32(int(v)) == v {
-		return int(v)
+	v := int(fr.maxHeaderListSize())
+	if v < 0 {
+		// If maxHeaderListSize overflows an int, use no limit (0).
+		return 0
 	}
-	// They had a crazy big number for MaxHeaderBytes anyway,
-	// so give them unlimited header lengths:
-	return 0
+	return v
 }
 
 // readMetaFrame returns 0 or more CONTINUATION frames from fr and