QCoffPeParser: simplify std::optional check

Even in C++98 one could declare variables in if statements and
contextually convert them to bool:

   if (D* d = dynamic_cast<D*>(base))
      ~~~
   else if (D2* d = dynamic_cast<D2*>(base))
      ~~~

std::optional is contextually convertible to bool, so we don't need
C++17 if initializers. Just use

   if (auto optional = ~~~~)

Pick-to: 6.3
Change-Id: Ie64c2c1ceb7b608eda0534d837f46d3c4be49bf8
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/plugin/qcoffpeparser.cpp

@@ -364,7 +364,7 @@ QLibraryScanResult QCoffPeParser::parse(QByteArrayView data, QString *errMsg)
         return {};
 
     QByteArrayView stringTable;
-    if (auto optional = findStringTable(data, ntHeaders, error); optional)
+    if (auto optional = findStringTable(data, ntHeaders, error))
         stringTable = *optional;
     else
         return {};