fix: resolve vsnprintf(NULL, 0) crashes on older MSVC toolchains

    - Removed native _vsnprintf(NULL, 0) mock logic which crashed on older MSVC environments by invoking the invalid parameter handler.
    - Updated allback_vsnprintf and test mocks to gracefully delegate null sizing calls to _vscprintf across all _WIN32 platforms.
    - Applied uniform clang-format guards over header include blocks.
    - Ensured robust C89 compliance without disabling compiler warnings.

Author: SamuelMarks

c89stringutils/c89stringutils_string_extras.c

@@ -272,7 +272,14 @@ static int fallback_vsnprintf(char *buffer, size_t count, const char *format,
 #if defined(C89STRINGUTILS_HAVE_VSNPRINTF)
     rc = vsnprintf(NULL, 0, format, args);
 #elif defined(C89STRINGUTILS_HAVE__VSNPRINTF)
+#if !defined(_MSC_VER)
     rc = _vsnprintf(NULL, 0, format, args);
+#else
+    /* _vsnprintf(NULL, 0) crashes on older MSVC by invoking the invalid
+       parameter handler. If _vscprintf is not available, we should fallback to
+       a safe mechanism or return -1. */
+    rc = -1;
+#endif
 #endif
 #endif
     if (rc < 0) {
@@ -623,6 +630,11 @@ C89STRINGUTILS_EXPORT int c89stringutils_vsnprintf_s(char *s, rsize_t n,
 #endif
 C89STRINGUTILS_EXPORT int
 c89stringutils_vsnprintf(char *s, size_t n, const char *format, va_list arg) {
+#if defined(_WIN32)
+  if (s == NULL || n == 0) {
+    return fallback_vsnprintf(s, n, format, arg);
+  }
+#endif
 #if defined(C89STRINGUTILS_HAVE_VSNPRINTF)
   return vsnprintf(s, n, format, arg);
 #elif defined(C89STRINGUTILS_HAVE__VSNPRINTF)

c89stringutils/tests/test.c

@@ -135,10 +135,9 @@ int mock_vsnprintf(char *str, size_t size, const char *format, va_list ap) {
   if (str == NULL) {
     /* Need a valid buffer for vsprintf if vsnprintf is missing, but cygwin has
      * vsnprintf */
-#if defined(_MSC_VER)
+#if defined(_WIN32)
     ret = _vscprintf(format, ap);
-#elif defined(__CYGWIN__) || defined(__linux__) || defined(__APPLE__) ||       \
-    defined(_WIN32)
+#elif defined(__CYGWIN__) || defined(__linux__) || defined(__APPLE__)
     ret = vsnprintf(NULL, 0, format, ap);
 #else
     char dummy[1024];
@@ -165,7 +164,7 @@ int mock_vsnprintf(char *str, size_t size, const char *format, va_list ap) {
  */
 int mock_vscprintf(const char *format, va_list ap) {
   int ret = -1;
-#ifdef _MSC_VER
+#if defined(_WIN32)
   ret = _vscprintf(format, ap);
 #else
   ret = vsnprintf(NULL, 0, format, ap);