Fix wrong resulting string length in UTF-16 to wchar_t conversion.

Don't optimize the returned length for surrogate-less case, this does save a
pass of the string but at the price of returning a wrong result, which is not
worth it, just compute the really required length exactly.

Closes #16298.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@76622 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

tests/strings/unicode.cpp

@@ -390,6 +390,11 @@ void UnicodeTestCase::ConversionUTF16()
     size_t len;
     conv.cMB2WC("\x01\0\0B\0C" /* A macron BC */, 6, &len);
     CPPUNIT_ASSERT_EQUAL( 3, len );
+
+    // Another one: verify that the length of the resulting string is computed
+    // correctly when there is a surrogate in the input.
+    wxMBConvUTF16BE().cMB2WC("\xd8\x03\xdc\x01" /* OLD TURKIC LETTER YENISEI A */, wxNO_LEN, &len);
+    CPPUNIT_ASSERT_EQUAL( 1, len );
 }
 
 void UnicodeTestCase::ConversionUTF32()