Handle case when compiler reports incorrect node location

fixes #168

spec/ameba/ast/util_spec.cr

@@ -60,6 +60,24 @@ module Ameba::AST
           "          end",
         ])
       end
+
+      it "does not report source of node which has incorrect location" do
+        s = %q(
+          module MyModule
+            macro conditional_error_for_inline_callbacks
+              \{%
+                raise ""
+              %}
+            end
+
+            macro before_save(x = nil)
+            end
+          end
+        )
+        node = as_nodes(s).nil_literal_nodes.first
+        source = subject.node_source node, s.split("\n")
+        source.should be_nil
+      end
     end
 
     describe "#flow_command?" do

spec/ameba/rule/lint/empty_expression_spec.cr

@@ -95,6 +95,22 @@ module Ameba
       end
     )
 
+    it "does not report emtpy expression in macro" do
+      s = Source.new %q(
+        module MyModule
+          macro conditional_error_for_inline_callbacks
+            \{%
+              raise ""
+            %}
+          end
+
+          macro before_save(x = nil)
+          end
+        end
+      )
+      subject.catch(s).should be_valid
+    end
+
     it "reports rule, location and message" do
       s = Source.new %(
         if ()

spec/spec_helper.cr

@@ -151,6 +151,7 @@ module Ameba
 
   class TestNodeVisitor < Crystal::Visitor
     NODES = [
+      Crystal::NilLiteral,
       Crystal::Var,
       Crystal::Assign,
       Crystal::OpAssign,

src/ameba/ast/util.cr

@@ -40,7 +40,8 @@ module Ameba::AST::Util
     node_lines = code_lines[line..end_line]
     first_line, last_line = node_lines[0]?, node_lines[-1]?
 
-    return unless first_line && last_line
+    return if first_line.nil? || last_line.nil?
+    return if first_line.size < column # compiler reports incorrection location
 
     node_lines[0] = first_line.sub(0...column, "")
 
@@ -48,6 +49,8 @@ module Ameba::AST::Util
       end_column = end_column - column
       last_line = node_lines[0]
     end
+
+    return if last_line.size < end_column + 1
     node_lines[-1] = last_line.sub(end_column + 1...last_line.size, "")
 
     node_lines