Use val.at instead of val[]

libqpdf/QPDF_String.cc

@@ -184,9 +184,9 @@ QPDF_String::getUTF8Val() const
         return QUtil::utf16_to_utf8(this->val);
     }
     else if ((val.length() >= 3) &&
-             (val[0] == '\xEF') &&
-             (val[1] == '\xBB') &&
-             (val[2] == '\xBF'))
+             (val.at(0) == '\xEF') &&
+             (val.at(1) == '\xBB') &&
+             (val.at(2) == '\xBF'))
     {
         // PDF 2.0 allows UTF-8 strings when explicitly prefixed with
         // the above bytes, which is just UTF-8 encoding of U+FEFF.