fix: check for valid buffer handles in modify_keymap (#13543)

Fixes #13541
Neovim would crash when trying to map a key on non existant buffer

test/functional/api/keymap_spec.lua

@@ -809,4 +809,9 @@ describe('nvim_buf_set_keymap, nvim_buf_del_keymap', function()
     command('normal lhs')
     eq({'rhs'}, bufmeths.get_lines(0, 0, 1, 1))
   end)
+
+  it("does not crash when setting keymap in a non-existing buffer #13541", function()
+    pcall_err(bufmeths.set_keymap, 100, '', 'lsh', 'irhs<Esc>', {})
+    helpers.assert_alive()
+  end)
 end)