include/msvcrt: Define _TRUNCATE; use it in the tests.

dlls/msvcrt/tests/string.c

@@ -1215,22 +1215,22 @@ static void test_mbstowcs(void)
         return;
     }
 
-    err = pmbstowcs_s(&ret, wOut, 6, mSimple, -1/*_TRUNCATE*/);
+    err = pmbstowcs_s(&ret, wOut, 6, mSimple, _TRUNCATE);
     ok(err == 0, "err = %d\n", err);
     ok(ret == 5, "ret = %d\n", (int)ret);
     ok(!memcmp(wOut, wSimple, sizeof(wSimple)), "wOut = %s\n", wine_dbgstr_w(wOut));
 
-    err = pmbstowcs_s(&ret, wOut, 6, mHiragana, -1/*_TRUNCATE*/);
+    err = pmbstowcs_s(&ret, wOut, 6, mHiragana, _TRUNCATE);
     ok(err == 0, "err = %d\n", err);
     ok(ret == 3, "ret = %d\n", (int)ret);
     ok(!memcmp(wOut, wHiragana, sizeof(wHiragana)), "wOut = %s\n", wine_dbgstr_w(wOut));
 
-    err = pwcstombs_s(&ret, mOut, 6, wSimple, -1/*_TRUNCATE*/);
+    err = pwcstombs_s(&ret, mOut, 6, wSimple, _TRUNCATE);
     ok(err == 0, "err = %d\n", err);
     ok(ret == 5, "ret = %d\n", (int)ret);
     ok(!memcmp(mOut, mSimple, sizeof(mSimple)), "mOut = %s\n", mOut);
 
-    err = pwcstombs_s(&ret, mOut, 6, wHiragana, -1/*_TRUNCATE*/);
+    err = pwcstombs_s(&ret, mOut, 6, wHiragana, _TRUNCATE);
     ok(err == 0, "err = %d\n", err);
     ok(ret == 5, "ret = %d\n", (int)ret);
     ok(!memcmp(mOut, mHiragana, sizeof(mHiragana)), "mOut = %s\n", mOut);

include/msvcrt/stdlib.h

@@ -47,6 +47,8 @@ typedef struct
 #define _MAX_PATH           260
 #endif
 
+/* Make the secure string functions (names end in "_s") truncate their output */
+#define _TRUNCATE  ((size_t)-1)
 
 typedef struct _div_t {
     int quot;