fix strtod int optimization in non-nearest rounding mode

the mid-sized integer optimization relies on lnz set up properly
to mark the last non-zero decimal digit, but this was not done
if the non-zero digit lied outside the KMAX digits of the base
10^9 number representation.

so if the fractional part was a very long list of zeros (>2048*9 on
x86) followed by non-zero digits then the integer optimization could
kick in discarding the tiny non-zero fraction which can mean wrong
result on non-nearest rounding mode.

strtof, strtod and strtold were all affected.

src/internal/floatscan.c

@@ -110,7 +110,10 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 			gotdig=1;
 		} else {
 			dc++;
-			if (c!='0') x[KMAX-4] |= 1;
+			if (c!='0') {
+				lnz = dc;
+				x[KMAX-4] |= 1;
+			}
 		}
 	}
 	if (!gotrad) lrp=dc;