Bugfix: missing return statement in custom ostream operator<< added

When using clang, writing to output stream caused segmentation violation
because the stream object was not returned by the operator which has been
used just before.
Adding also some other return statements (though irrelevant for the crash).

Crash log
* thread #1: tid = 0x597d9, 0x0000000100001f7f testMessageList`void print_list<AliceO2::Format::messageList<TestMsg, SimpleHeader_t> >(list=<unavailable>, hdrsel=<unavailable>) + 191 at testMessageList.cxx:77, queue = 'com.apple.main-thread', stop reason = EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)
    frame #0: 0x0000000100001f7f testMessageList`void print_list<AliceO2::Format::messageList<TestMsg, SimpleHeader_t> >(list=<unavailable>, hdrsel=<unavailable>) + 191 at testMessageList.cxx:77
   74          it != list.end();
   75          ++it) {
   76       // the iterator defines a conversion operator to the header type
-> 77       std::cout << static_cast<typename ListType::header_type>(it) << std::end;
   78       // dereferencing of the iterator gives the payload
   79       std::cout << *it << std::end;
   80     }

Author: matthiasrichter

DataFormats/Generic/message_list.h

@@ -56,6 +56,8 @@ class messageList {
     const HdrT* srcHeader = reinterpret_cast<const HdrT*>(headerData);
     // TODO: consistency check
     mDataArray.push_back(messagePair(*srcHeader, payloadMsg));
+
+    return mDataArray.size();
   }
   /** number of data blocks in the list */
   size_t size() {return mDataArray.size();}

DataFormats/Generic/test/testMessageList.cxx

@@ -31,6 +31,8 @@ struct SimpleHeader_t {
 std::ostream& operator<<(std::ostream& stream, SimpleHeader_t header) {
   stream << "Header ID: " << header.id << std::endl;
   stream << "Header Specification: " << std::hex << header.specification;
+
+  return stream;
 }
 
 // more complex message type, some class which wraps around payload
@@ -45,6 +47,8 @@ class TestMsg {
   ~TestMsg() {clear();}
 
   int alloc(int size) {
+    // not yet implemented
+    return 0;
   }
 
   uint8_t* get() const {