Check _WIN32 and WINDOWS instaead of __MSC_VER
Check __WIN32__ in string_format, too.

Also increase the buffer size to make vsnprint succeed more likely.
Remove debug message from vstring_format,

The returned string should indicate the problematic format string.
Merge pull request #6547 from BevapDin/fix-string-format

Fix the "invalid input to string_format function!" message