Fix (v)sprintf

9f5a68e789 changed its behavior to append
rather than set the output string. Which is misleading. This reverts
(v)sprintf back to set the string.

Signed-off-by: Simon Rozman <simon@rozman.si>

include/WinStd/Common.h

@@ -261,7 +261,7 @@ static int vsprintf(_Inout_ std::basic_string<char, _Traits, _Ax> &str, _In_z_ _
     int count = _vsnprintf(buf, _countof(buf), format, arg);
     if (0 <= count && count < _countof(buf)) {
         // Copy from stack.
-        str.append(buf, count);
+        str.assign(buf, count);
         return count;
     }
     if (count < 0) {
@@ -275,9 +275,8 @@ static int vsprintf(_Inout_ std::basic_string<char, _Traits, _Ax> &str, _In_z_ _
         default: throw std::runtime_error("failed to format string");
         }
     }
-    size_t offset = str.size();
-    str.resize(offset + count);
-    if (_vsnprintf(&str[offset], static_cast<size_t>(count) + 1, format, arg) != count)
+    str.resize(count);
+    if (_vsnprintf(&str[0], static_cast<size_t>(count) + 1, format, arg) != count)
         throw std::runtime_error("failed to format string");
     return count;
 }
@@ -300,7 +299,7 @@ static int vsprintf(_Inout_ std::basic_string<wchar_t, _Traits, _Ax> &str, _In_z
     int count = _vsnwprintf(buf, _countof(buf), format, arg);
     if (0 <= count && count < _countof(buf)) {
         // Copy from stack.
-        str.append(buf, count);
+        str.assign(buf, count);
         return count;
     }
     if (count < 0) {
@@ -314,9 +313,8 @@ static int vsprintf(_Inout_ std::basic_string<wchar_t, _Traits, _Ax> &str, _In_z
         default: throw std::runtime_error("failed to format string");
         }
     }
-    size_t offset = str.size();
-    str.resize(offset + count);
-    if (_vsnwprintf(&str[offset], static_cast<size_t>(count) + 1, format, arg) != count)
+    str.resize(count);
+    if (_vsnwprintf(&str[0], static_cast<size_t>(count) + 1, format, arg) != count)
         throw std::runtime_error("failed to format string");
     return count;
 }