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[lib/c] Fix number conversions with prefix

When we want to prefix with 0 a number we have to worry about the
prefix of the number, because if it begins with a sign or 0x then
we have to append the 0 after the prefix.

--- a/lib/c/src/vfprintf.c

+++ b/lib/c/src/vfprintf.c

@@ -80,6 +80,7 @@

 {

 	char *buf0 = buf;

 	int len, base = conv->base, prec = conv->prec;

+	uintmax_t oval = val;

 	if (prec == -1)

 		prec = 1;

@@ -89,14 +90,10 @@

 	while (buf0 - buf < prec)

 		*--buf = '0';

-	/*

-	 * TODO: It cannot be done here because the combination

-	 * %#04x produces 00x1

-	 */

 	if (flags & ALTFORM) {

 		if (base == 8 && *buf != '0') {

 			*--buf = '0';

-		} else if (base == 16 && val != 0) {

+		} else if (base == 16 && oval != 0) {

 			*--buf = conv->digs[16];

 			*--buf = '0';

 		}

@@ -161,7 +158,7 @@

 static size_t

 strout(char *s, size_t len, int width, int fill, FILE * restrict fp)

 {

-	int left = 0, adjust, ch;

+	int left = 0, adjust, ch, prefix;

 	size_t cnt = 0;

 	if (width < 0) {

@@ -175,6 +172,19 @@

 	cnt = adjust + len;

 	if (left)

 		adjust = -adjust;

+

+	if (fill == '0') {

+		if (*s == '-' || *s == '+')

+			prefix = 1;

+		else if (*s == '0' && toupper(s[1]) == 'X')

+			prefix = 2;

+		else

+			prefix = 0;

+		while (prefix--) {

+			putc(*s++, fp);

+			--len;

+		}

+	}

 	for ( ; adjust > 0; adjust--)

 		putc(fill, fp);