Fix uninitialized member warnings

Based on PVS Studio report

pcbnew/pcb_expr_evaluator.cpp

@@ -1073,11 +1073,15 @@ PCB_EXPR_COMPILER::PCB_EXPR_COMPILER()
 }
 
 
-PCB_EXPR_EVALUATOR::PCB_EXPR_EVALUATOR()
+PCB_EXPR_EVALUATOR::PCB_EXPR_EVALUATOR() :
+    m_result( 0 ),
+    m_compiler(),
+    m_ucode(),
+    m_errorStatus()
 {
-    m_result = 0;
 }
 
+
 PCB_EXPR_EVALUATOR::~PCB_EXPR_EVALUATOR()
 {
 }