Make error checking in QSignalSpy consistent

QSignalSpy's constructor failed gracefully for some problems with the
parameters, but not for null parameters, for which there was only a
Q_ASSERT.  This commit makes the handling of null parameters consistent
with the handling of other errors -- output a meaningful error message
with qWarning() and return, so that isValid() will subsequently return
false.

Change-Id: I7f5677a4c10185e30403ce3e12a022de8c13bc1c
Task-number: QTBUG-14283
Reviewed-by: Rohan McGovern

src/testlib/qsignalspy.h

@@ -68,8 +68,15 @@ public:
 #else
         static const int memberOffset = QObject::staticMetaObject.methodCount();
 #endif
-        Q_ASSERT(obj);
-        Q_ASSERT(aSignal);
+        if (!obj) {
+            qWarning("QSignalSpy: Cannot spy on a null object");
+            return;
+        }
+
+        if (!aSignal) {
+            qWarning("QSignalSpy: Null signal name is not valid");
+            return;
+        }
 
         if (((aSignal[0] - '0') & 0x03) != QSIGNAL_CODE) {
             qWarning("QSignalSpy: Not a valid signal, use the SIGNAL macro");

src/testlib/qsignalspy.qdoc

@@ -62,7 +62,11 @@
 /*! \fn QSignalSpy::QSignalSpy(QObject *object, const char *signal)
 
     Constructs a new QSignalSpy that listens for emissions of the \a signal
-    from the QObject \a object. Neither \a signal nor \a object can be null.
+    from the QObject \a object. If QSignalSpy is not able to listen for a
+    valid signal (for example, because \a object is null or \a signal does
+    not denote a valid signal of \a object), an explanatory warning message
+    will be output using qWarning() and subsequent calls to \c isValid() will
+    return false.
 
     Example:
     \snippet doc/src/snippets/code/doc_src_qsignalspy.cpp 4