Fix potential recursive calls of WarpX constructor. (#6223)

WarpX's constructor may build objects that call WarpX::getInstance,
which in turn build WarpX again if `m_instance` is nullptr. So we should
assign `this` to `m_instance` at the beginning of the constructor to
avoid the recursion.

Author: WeiqunZhang

Source/WarpX.cpp

@@ -320,6 +320,9 @@ WarpX::Finalize()
 
 WarpX::WarpX ()
 {
+    m_instance = this; // This guarantees that GetInstance() can be
+                       // indirectly used in WarpX constructor.
+
     warpx::initialization::initialize_warning_manager();
 
     ReadParameters();