Fix bug with non-NUL-terminaed inputs in wxMBConvUTF8.

We read beyond the provided maximal length as we didn't update the remaining
length while parsing the remaining bytes of an UTF-8-encoded code point.

Fix this and add a test for it.

Closes #15901.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@75733 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

src/common/strconv.cpp

@@ -1272,6 +1272,14 @@ size_t wxMBConvUTF8::ToWChar(wchar_t *buf, size_t n,
                 wxUint32 res = cc & (0x3f >> cnt);
                 while (cnt--)
                 {
+                    if (!isNulTerminated && !srcLen)
+                    {
+                        // invalid UTF-8 sequence ending before the end of code
+                        // point.
+                        invalid = true;
+                        break;
+                    }
+
                     cc = *psz;
                     if ((cc & 0xC0) != 0x80)
                     {
@@ -1281,6 +1289,8 @@ size_t wxMBConvUTF8::ToWChar(wchar_t *buf, size_t n,
                     }
 
                     psz++;
+                    if (!isNulTerminated)
+                        srcLen--;
                     res = (res << 6) | (cc & 0x3f);
                 }