simplify/debloat radix point alignment code in floatscan

now that this is the first operation, it can rely on the circular
buffer contents not being wrapped when it begins. we limit the number
of digits read slightly in the initial parsing loops too so that this
code does not have to consider the case where it might cause the
circular buffer to wrap; this is perfectly fine because KMAX is chosen
as a power of two for circular-buffer purposes and is much larger than
it otherwise needs to be, anyway.

these changes should not affect performance at all.

src/internal/floatscan.c

@@ -77,7 +77,7 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 		if (c == '.') {
 			if (lrp!=-1) break;
 			lrp = dc;
-		} else if (k < KMAX) {
+		} else if (k < KMAX-2) {
 			dc++;
 			if (j) x[k] = x[k]*10 + c-'0';
 			else x[k] = c-'0';
@@ -88,7 +88,7 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 			gotdig=1;
 		} else {
 			dc++;
-			x[KMAX-1] |= c-'0';
+			if (c!='0') x[KMAX-3] |= 1;
 		}
 	}
 	if (lrp==-1) lrp=dc;
@@ -146,7 +146,7 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 		int rpm9 = rp>=0 ? rp%9 : rp%9+9;
 		int p10 = p10s[rpm9-1];
 		uint32_t carry = 0;
-		for (k=a; k!=z; k=(k+1 & MASK)) {
+		for (k=a; k!=z; k++) {
 			uint32_t tmp = x[k] % p10;
 			x[k] = x[k]/p10 + carry;
 			carry = 1000000000/p10 * tmp;
@@ -155,12 +155,7 @@ static long double decfloat(FILE *f, int c, int bits, int emin, int sign, int po
 				rp -= 9;
 			}
 		}
-		if (carry) {
-			if ((z+1 & MASK) != a) {
-				x[z] = carry;
-				z = (z+1 & MASK);
-			} else x[z-1 & MASK] |= 1;
-		}
+		if (carry) x[z++] = carry;
 		rp += 9-rpm9;
 	}