fix Path::ext() when new_ext == ""

test/src/test_Path.cc

@@ -49,6 +49,7 @@ TEST(PathTest, ext)
     EXPECT_EQ("file.h", Path("file.c").ext("h")->to_s());
     EXPECT_EQ("/some/crazy/dir/foo.txt", Path("/some/crazy/dir/foo").ext(".txt")->to_s());
     EXPECT_EQ("/some/crazy/dir/foo.txt", Path("/some/crazy/dir/foo").ext("txt")->to_s());
+    EXPECT_EQ("C:/baz", Path("C:\\baz.xyz").ext("")->to_s());
 }
 
 TEST(PathTest, exists)