fix on eval with not exsisting file.

`eval` can specify unexisting file so it shouldn't read.
Without `eval` the file can be removed just after compiling,
the error should not be propagated.
If given file is existing file for eval context, it should not
override the source code.

The issue is pointed at #10 by
casperisfine and this patc contains a part of his patch.

Author: ko1

lib/debug/source_repository.rb

@@ -9,8 +9,16 @@ def add iseq, src
       path = '-e' if iseq.path == '-e'
 
       case
+      when src
+        if  File.file?(path)
+          path = '(eval)' + path
+          src = nil
+        end
       when path = iseq.absolute_path
-        src = File.read(path)
+        begin
+          src = File.read(path)
+        rescue SystemCallError
+        end
       when iseq.path == '-e'
         path = '-e'
       else