Prevent an "instance variable not initialized" warning

Under $VERBOSE=true, simplecov emits the following warning:

```
lib/simplecov/configuration.rb:203: warning: instance variable @enable_for_subprocesses not initialized
```

This change suppresses the warning.

Author: mame

lib/simplecov/configuration.rb

@@ -199,8 +199,9 @@ def at_exit(&block)
     # gets or sets the enabled_for_subprocess configuration
     # when true, this will inject SimpleCov code into Process.fork
     def enable_for_subprocesses(value = nil)
-      @enable_for_subprocesses = value unless value.nil?
-      @enable_for_subprocesses || false
+      return @enable_for_subprocesses if defined?(@enable_for_subprocesses) && value.nil?
+
+      @enable_for_subprocesses = value || false
     end
 
     # gets the enabled_for_subprocess configuration