Guarantee level_override exists

Some Ruby apps subclass Logger without running the superclass
constructor, which means that `@level_override` isn't initialized
properly. This can be fixed in some cases, but the gem should maintain
backwards compatibility.

lib/logger.rb

@@ -381,7 +381,7 @@ class Logger
 
   # Logging severity threshold (e.g. <tt>Logger::INFO</tt>).
   def level
-    @level_override[Fiber.current] || @level
+    level_override[Fiber.current] || @level
   end
 
   # Sets the log level; returns +severity+.
@@ -406,14 +406,14 @@ def level=(severity)
   #     logger.debug { "Hello" }
   #   end
   def with_level(severity)
-    prev, @level_override[Fiber.current] = level, Severity.coerce(severity)
+    prev, level_override[Fiber.current] = level, Severity.coerce(severity)
     begin
       yield
     ensure
       if prev
-        @level_override[Fiber.current] = prev
+        level_override[Fiber.current] = prev
       else
-        @level_override.delete(Fiber.current)
+        level_override.delete(Fiber.current)
       end
     end
   end
@@ -746,6 +746,11 @@ def format_severity(severity)
     SEV_LABEL[severity] || 'ANY'
   end
 
+  # Guarantee the existence of this ivar even when subclasses don't call the superclass constructor.
+  def level_override
+    @level_override ||= {}
+  end
+
   def format_message(severity, datetime, progname, msg)
     (@formatter || @default_formatter).call(severity, datetime, progname, msg)
   end