Make sure to specify buffer size large enough for NULL terminator

renderdoc/os/win32/win32_stringio.cpp

@@ -303,10 +303,9 @@ namespace StringFormat
 		int size = StringFormat::vsnprintf(NULL, 0, format, args2);
 
 		char *buf = new char[size+1];
+		StringFormat::vsnprintf(buf, size+1, format, args);
 		buf[size] = 0;
 
-		StringFormat::vsnprintf(buf, size, format, args);
-
 		va_end(args);
 		va_end(args2);