Ensure PID is an integer

This ensures that the `ps` call doesn't invoke something else.

lib/get_process_mem.rb

@@ -48,7 +48,7 @@ class GetProcessMem
   def initialize(pid = Process.pid)
     @status_file  = Pathname.new "/proc/#{pid}/status"
     @process_file = Pathname.new "/proc/#{pid}/smaps"
-    @pid          = pid
+    @pid          = Integer(pid)
     @linux        = @status_file.exist?
   end
 

test/get_process_mem_test.rb

@@ -15,6 +15,15 @@ def test_different_pid_returns_different_memory
     Process.wait(pid) if pid
   end
 
+  def test_invalid_pid
+    raised_exception = false
+    GetProcessMem.new('ls')
+  rescue ArgumentError
+    raised_exception = true
+  ensure
+    assert raised_exception
+  end
+
   def test_seems_to_work
     assert @mem.kb    > 0
     assert @mem.mb    > 0