Avoid possible null pointer access.

src/dfmt/ast_info.d

@@ -209,9 +209,12 @@ final class FormatVisitor : ASTVisitor
     override void visit (const FunctionBody functionBody)
     {
         if (auto bd = functionBody.specifiedFunctionBody)
+        {
+            if (bd.blockStatement)
             {
                 astInformation.funBodyLocations ~= bd.blockStatement.startLocation;
             }
+        }
         functionBody.accept(this);
     }