fix printf rounding with %g for some corner case midpoints

the subsequent rounding code assumes the end pointer (z) accurately
reflects the end of significance in the decimal expansion, but for
certain large integers, spurious trailing zero slots were left behind
when applying the binary exponent.

issue reported by Morten Welinder; the analysis of the cause was
performed by nsz, who also proposed this change.

src/stdio/vfprintf.c

@@ -308,8 +308,8 @@ static int fmt_fp(FILE *f, long double y, int w, int p, int fl, int t)
 			*d = x % 1000000000;
 			carry = x / 1000000000;
 		}
-		if (!z[-1] && z>a) z--;
 		if (carry) *--a = carry;
+		while (z>a && !z[-1]) z--;
 		e2-=sh;
 	}
 	while (e2<0) {