fmt: catch overflow in width and prec calculations

Fixes #10674.

Change-Id: If3fae3244d87aeaa70815f499105c264394aa7ad
Reviewed-on: https://go-review.googlesource.com/9657Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/fmt_test.go

@@ -537,6 +537,7 @@ var fmtTests = []struct {
 	{"%s", nil, "%!s(<nil>)"},
 	{"%T", nil, "<nil>"},
 	{"%-1", 100, "%!(NOVERB)%!(EXTRA int=100)"},
+	{"%017091901790959340919092959340919017929593813360", 0, "%!(NOVERB)%!(EXTRA int=0)"},
 
 	// The "<nil>" show up because maps are printed by
 	// first obtaining a list of keys and then looking up

src/fmt/print.go

@@ -292,6 +292,9 @@ func parsenum(s string, start, end int) (num int, isnum bool, newi int) {
 	}
 	for newi = start; newi < end && '0' <= s[newi] && s[newi] <= '9'; newi++ {
 		num = num*10 + int(s[newi]-'0')
+		if num < 0 {
+			return 0, false, end // Overflow; crazy long number most likely.
+		}
 		isnum = true
 	}
 	return