patch 8.2.0418: code in eval.c not sufficiently covered by tests

Problem:    Code in eval.c not sufficiently covered by tests.
Solution:   Add more tests. (Yegappan Lakshmanan, closes #5815)
diff --git a/src/testdir/test_blob.vim b/src/testdir/test_blob.vim
index 79fdf44..fb56173 100644
--- a/src/testdir/test_blob.vim
+++ b/src/testdir/test_blob.vim
@@ -87,6 +87,7 @@
   call assert_equal(0z0011223344, b[:])
   call assert_equal(0z0011223344, b[:-1])
   call assert_equal(0z, b[5:6])
+  call assert_equal(0z0011, b[-10:1])
 endfunc
 
 func Test_blob_get()