fix strtod and strtof rounding with many trailing zeros

in certain cases excessive trailing zeros could cause incorrect
rounding from long double to double or float in decfloat.

e.g. in strtof("9444733528689243848704.000000", 0) the argument
is 0x1.000001p+73, exactly halfway between two representible floats,
this incorrectly got rounded to 0x1.000002p+73 instead of 0x1p+73,
but with less trailing 0 the rounding was fine.

the fix makes sure that the z index always points one past the last
non-zero digit in the base 10^9 representation, this way trailing
zeros don't affect the rounding logic.

src/internal/floatscan.c

@@ -172,6 +172,9 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 			return sign * (long double)x[0] * p10s[rp-10];
 	}
 
+	/* Drop trailing zeros */
+	for (; !x[z-1]; z--);
+
 	/* Align radix point to B1B digit boundary */
 	if (rp % 9) {
 		int rpm9 = rp>=0 ? rp%9 : rp%9+9;