patch 8.2.1678: crash when using ":set" after ":ownsyntax"

Problem:    Crash when using ":set" after ":ownsyntax".
Solution:   Make sure 'spelloptions' is not NULL. (closes #6950)

Author: brammool

src/syntax.c

@@ -6316,9 +6316,11 @@ ex_ownsyntax(exarg_T *eap)
 #ifdef FEAT_SPELL
 	// TODO: keep the spell checking as it was.
 	curwin->w_p_spell = FALSE;	// No spell checking
+	// make sure option values are "empty_option" instead of NULL
 	clear_string_option(&curwin->w_s->b_p_spc);
 	clear_string_option(&curwin->w_s->b_p_spf);
 	clear_string_option(&curwin->w_s->b_p_spl);
+	clear_string_option(&curwin->w_s->b_p_spo);
 #endif
 	clear_string_option(&curwin->w_s->b_syn_isk);
     }

src/testdir/test_syntax.vim

@@ -428,7 +428,11 @@ func Test_ownsyntax()
   call setline(1, '#define FOO')
   syntax on
   set filetype=c
+
   ownsyntax perl
+  " this should not crash
+  set
+
   call assert_equal('perlComment', synIDattr(synID(line('.'), col('.'), 1), 'name'))
   call assert_equal('c',    b:current_syntax)
   call assert_equal('perl', w:current_syntax)

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1678,
 /**/
     1677,
 /**/