patch 9.0.0057: has('patch-xxx') returns true

Problem:    has('patch-xxx') returns true.
Solution:   Check for digit. (closes #10751)
diff --git a/src/testdir/test_expr.vim b/src/testdir/test_expr.vim
index ffac6df..f911cd9 100644
--- a/src/testdir/test_expr.vim
+++ b/src/testdir/test_expr.vim
@@ -41,6 +41,7 @@
   call assert_false(has('patch-7.4.'))
   call assert_false(has('patch-9.1.0'))
   call assert_false(has('patch-9.9.1'))
+  call assert_false(has('patch-abc'))
 endfunc
 
 func Test_op_ternary()