fix: fixed salary structure assignment patch

[nabinhait]

Executing erpnext.patches.v11_0.create_salary_structure_assignments in site1.local (1bd3e0294d)
Traceback (most recent call last):
  File "/usr/lib/python2.7/runpy.py", line 162, in _run_module_as_main
    "__main__", fname, loader, pkg_name)
  File "/usr/lib/python2.7/runpy.py", line 72, in _run_code
    exec code in run_globals
  File "/home/frappe/frappe-bench/apps/frappe/frappe/utils/bench_helper.py", line 97, in <module>
    main()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/utils/bench_helper.py", line 18, in main
    click.Group(commands=commands)(prog_name='bench')
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 764, in __call__
    return self.main(*args, **kwargs)
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 717, in main
    rv = self.invoke(ctx)
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 1137, in invoke
    return _process_result(sub_ctx.command.invoke(sub_ctx))
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 956, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/core.py", line 555, in invoke
    return callback(*args, **kwargs)
  File "/home/frappe/frappe-bench/env/local/lib/python2.7/site-packages/click/decorators.py", line 17, in new_func
    return f(get_current_context(), *args, **kwargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/commands/__init__.py", line 25, in _func
    ret = f(frappe._dict(ctx.obj), *args, **kwargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/commands/site.py", line 227, in migrate
    migrate(context.verbose, rebuild_website=rebuild_website)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/migrate.py", line 38, in migrate
    frappe.modules.patch_handler.run_all()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/modules/patch_handler.py", line 29, in run_all
    if not run_single(patchmodule = patch):
  File "/home/frappe/frappe-bench/apps/frappe/frappe/modules/patch_handler.py", line 63, in run_single
    return execute_patch(patchmodule, method, methodargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/modules/patch_handler.py", line 83, in execute_patch
    frappe.get_attr(patchmodule.split()[0] + ".execute")()
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/patches/v11_0/create_salary_structure_assignments.py", line 49, in execute
    s.save()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 260, in save
    return self._save(*args, **kwargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 283, in _save
    self.insert()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 222, in insert
    self.run_before_save_methods()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 876, in run_before_save_methods
    self.run_method("validate")
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 772, in run_method
    out = Document.hook(fn)(self, *args, **kwargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 1048, in composer
    return composed(self, method, *args, **kwargs)
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 1031, in runner
    add_to_return_value(self, fn(self, *args, **kwargs))
  File "/home/frappe/frappe-bench/apps/frappe/frappe/model/document.py", line 766, in <lambda>
    fn = lambda self, *args, **kwargs: getattr(self, method)(*args, **kwargs)
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/hr/doctype/salary_structure_assignment/salary_structure_assignment.py", line 15, in validate
    self.validate_dates()
  File "/home/frappe/frappe-bench/apps/erpnext/erpnext/hr/doctype/salary_structure_assignment/salary_structure_assignment.py", line 27, in validate_dates
    .format(self.from_date, joining_date))
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 351, in throw
    msgprint(msg, raise_exception=exc, title=title, indicator='red')
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 337, in msgprint
    _raise_exception()
  File "/home/frappe/frappe-bench/apps/frappe/frappe/__init__.py", line 310, in _raise_exception
    raise raise_exception(msg)
frappe.exceptions.ValidationError: From Date 2017-12-04 cannot be before employee's joining Date 2018-08-26