fmt: fix panic with large precision

The code already handled high widths but not high precisions.
Also make sure it handles the harder cases of %U.

Fixes #10745.

Change-Id: Ib4d394d49a9941eeeaff866dc59d80483e312a98
Reviewed-on: https://go-review.googlesource.com/9769Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/fmt/fmt_test.go

@@ -557,6 +557,11 @@ var fmtTests = []struct {
 	{"%0.100f", 1.0, zeroFill("1.", 100, "")},
 	{"%0.100f", -1.0, zeroFill("-1.", 100, "")},
 
+	// Used to panic: integer function didn't look at f.prec or f.unicode.
+	{"%#.80x", 42, "0x0000000000000000000000000000000000000000000000000000000000000000000000000000002a"},
+	{"%.80U", 42, "U+0000000000000000000000000000000000000000000000000000000000000000000000000000002A"},
+	{"%#.80U", '日', "U+000000000000000000000000000000000000000000000000000000000000000000000000000065E5 '日'"},
+
 	// Comparison of padding rules with C printf.
 	/*
 		C program:

src/fmt/format.go

@@ -163,12 +163,20 @@ func (f *fmt) integer(a int64, base uint64, signedness bool, digits string) {
 	}
 
 	var buf []byte = f.intbuf[0:]
-	if f.widPresent {
-		width := f.wid
+	if f.widPresent || f.precPresent {
+		width := f.wid + f.prec // Only one will be set, both are positive; this provides the maximum.
 		if base == 16 && f.sharp {
 			// Also adds "0x".
 			width += 2
 		}
+		if f.unicode {
+			// Also adds "U+".
+			width += 2
+			if f.uniQuote {
+				// Also adds " 'x'".
+				width += 1 + 1 + utf8.UTFMax + 1
+			}
+		}
 		if width > nByte {
 			// We're going to need a bigger boat.
 			buf = make([]byte, width)