protect against recursing into handleResponse

those pesky nested event loops ...

i pondered various other scenarios (in particular, the adapter or
gdb crashing while the nested loop is running), but did not discover
serious side effects of it, so i'm not trying to handle it specially.

src/plugins/debugger/gdb/gdbengine.cpp

@@ -191,6 +191,9 @@ GdbEngine::GdbEngine(DebuggerManager *manager) :
     m_trkOptions->fromSettings(Core::ICore::instance()->settings());
     m_gdbAdapter = 0;
 
+    // Needs no resetting in initializeVariables()
+    m_busy = false;
+
     connect(theDebuggerAction(AutoDerefPointers), SIGNAL(valueChanged(QVariant)),
             this, SLOT(setAutoDerefPointers(QVariant)));
 }
@@ -594,6 +597,10 @@ void GdbEngine::readGdbStandardOutput()
 
     m_inbuffer.append(m_gdbProc.readAllStandardOutput());
 
+    // This can trigger when a dialog starts a nested event loop
+    if (m_busy)
+        return;
+
     while (newstart < m_inbuffer.size()) {
         int start = newstart;
         int end = m_inbuffer.indexOf('\n', scan);
@@ -612,7 +619,9 @@ void GdbEngine::readGdbStandardOutput()
                 continue;
         }
         #endif
+        m_busy = true;
         handleResponse(QByteArray::fromRawData(m_inbuffer.constData() + start, end - start));
+        m_busy = false;
     }
     m_inbuffer.clear();
 }