Replace QMap::operator[] with QMap::value()

QMap::operator[] creates a new default constructed entry in the map
if no entry with the given key exists. While not problematic (since
typically nullptrs are inserted) this is usually not what you want
for read access.

Signed-off-by: Berthold Stoeger <bstoeger@mail.tuwien.ac.at>

core/btdiscovery.cpp

@@ -46,7 +46,7 @@ static dc_descriptor_t *getDeviceType(QString btName)
 	}
 
 	if (!vendor.isEmpty() && !product.isEmpty())
-		return(descriptorLookup[vendor + product]);
+		return(descriptorLookup.value(vendor + product));
 
 	return(NULL);
 }