fix minor problem in previous strtod non-nearest rounding bug fix

commit 6ffdc4579f set lnz in the code
path for non-zero digits after a huge string of zeros, but the
assignment of dc to lnz truncates if the value of dc does not fit in
int; this is possible for some pathologically long inputs, either via
strings on 64-bit systems or via scanf-family functions.

instead, simply set lnz to match the point at which we add the
artificial trailing 1 bit to simulate nonzero digits after a huge
run of zeros.

src/internal/floatscan.c

@@ -111,7 +111,7 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 		} else {
 			dc++;
 			if (c!='0') {
-				lnz = dc;
+				lnz = (KMAX-4)*9;
 				x[KMAX-4] |= 1;
 			}
 		}