Fix wcpncpy test

tests/wchar/wcpncpy.c

@@ -4,15 +4,34 @@
 
 int main() {
     wchar_t src[] = L"Привет мир";
-    wchar_t dest[10];
-    wchar_t* result = wcpncpy(dest, src, 5);
 
-    assert(wcsncmp(dest, src, 5) == 0);
+    size_t n_input = 10;
+    size_t n_short = 6;
+    size_t n_long = 15;
 
-    // FIXME: seems to read from uninitialized memory?
-    // assert(*result == L'\0');
+    // Initialize with sentinel values to detect exactly how much is overwritten
+    wchar_t dest_short[] = L"\x12\x34\x56\x78\x90\x12\x34\x56\x78\x90\x12\x34\x56\x78";
+    wchar_t dest_long[] = L"\x12\x34\x56\x78\x90\x12\x34\x56\x78\x90\x12\x34\x56\x78";
 
-    assert(result == dest + 5);
+    wchar_t expected_short[] = L"Привет\x34\x56\x78\x90\x12\x34\x56\x78";
+
+    // The "short" test should copy exactly n_short characters without terminating null
+    wchar_t* result_short = wcpncpy(dest_short, src, n_short);
+    assert(wcsncmp(dest_short, src, n_short) == 0);
+    assert(result_short == dest_short + n_short);
+    for (size_t i = n_short; i < n_long; i++) {
+        // Check that the sentinel characters have not been overwritten
+        assert(dest_short[i] == expected_short[i]);
+    }
+
+    // The "long" test should write the input string, with nulls appended up to n_long
+    wchar_t* result_long = wcpncpy(dest_long, src, n_long);
+    assert(wcsncmp(dest_long, src, n_long) == 0);
+    assert(result_long == dest_long + n_input);
+    for (size_t i = n_input; i < n_long; i++) {
+        // Check that nulls are written up to n_long
+        assert(dest_long[i] == L'\0');
+    }
 
     return 0;
 }