QIsciiCodec: add an assert to silence Coverity

Coverity (not rightfully) complains that the code is using "iscii",
a uchar obtained by looking up into a table, as an index into the
"uni_to_iscii_pairs" array. Since the array is only 18 elements
long, there's the theoretic risk of accessing it past its end.

However, the lookup of "iscii" never returns values that may
actually go out of bounds. Coverity may be smart enough
to see the values that "iscii" can get and not raise the warning,
but since it does, make the code more robust and add an assert.

Task-number: QTBUG-43642
Change-Id: Id75ca105758b343102ca94137d0379c10e55581a
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/codecs/qisciicodec.cpp

@@ -212,6 +212,7 @@ QByteArray QIsciiCodec::convertFromUnicode(const QChar *uc, int len, ConverterSt
             if (iscii > 0x80) {
                 *ch++ = iscii;
             } else if (iscii) {
+                Q_ASSERT((2 * iscii) < (sizeof(uni_to_iscii_pairs) / sizeof(uni_to_iscii_pairs[0])));
                 const uchar *pair = uni_to_iscii_pairs + 2*iscii;
                 *ch++ = *pair++;
                 *ch++ = *pair++;