Don't overread or write memory returned by _DTOA_R

Don't over-read memory returned by _DTOA_R, and never write to it
since the result might be a string literal.

For example, when doing:
  swprintf(tt, 20, L"%.*f", 6, 0.0);

we will get back "0".

Instead, write the result returned by _DTOA_R to the output buffer.
After this, write the 0 chars directly to the the output buffer
(if there are any). This also has the (marginal) advantage that
we read/write less memory overall.

newlib/libc/stdio/vfwprintf.c

@@ -1631,12 +1631,18 @@ wcvt(struct _reent *data, _PRINTF_FLOAT_TYPE value, int ndigits, int flags,
 
 	{
 	  char *digits, *bp, *rve;
-#ifndef _MB_CAPABLE
 	  int i;
-#endif
 
 	  digits = _DTOA_R (data, value, mode, ndigits, decpt, &dsgn, &rve);
 
+#ifdef _MB_CAPABLE
+	  _mbsnrtowcs_r (data, buf, (const char **) &digits, rve - digits,
+			 len, NULL);
+#else
+	  for (i = 0; i < rve - digits && i < len; ++i)
+	    buf[i] = (wchar_t) digits[i];
+#endif
+
 	  if ((ch != L'g' && ch != L'G') || flags & ALT) {	/* Print trailing zeros */
 		bp = digits + ndigits;
 		if (ch == L'f' || ch == L'F') {
@@ -1646,18 +1652,13 @@ wcvt(struct _reent *data, _PRINTF_FLOAT_TYPE value, int ndigits, int flags,
 		}
 		if (value == 0)	/* kludge for __dtoa irregularity */
 			rve = bp;
-		while (rve < bp)
-			*rve++ = '0';
-	  }
 
+		for (i = rve - digits; i < bp - digits && i < len; ++i)
+			buf[i] = L'0';
+
+		rve = rve > bp ? rve : bp;
+	  }
 	  *length = rve - digits; /* full length of the string */
-#ifdef _MB_CAPABLE
-	  _mbsnrtowcs_r (data, buf, (const char **) &digits, *length,
-			 len, NULL);
-#else
-	  for (i = 0; i < *length && i < len; ++i)
-	    buf[i] = (wchar_t) digits[i];
-#endif
 	  return buf;
 	}
 }