spell: fix refresh of bar item "spell_suggest" when the input becomes empty (issue #1586)

When the input is empty, length of string is zero: when sending zero to
function weechat_string_dyn_alloc, the function returns NULL and therefore we
return immediately instead of handling the empty input, which is a valid value.

The regression was introduced by the use of dynamic strings, commit:
299f74bfef

(cherry picked from commit eb90a73fe8)

src/plugins/spell/spell.c

@@ -802,7 +802,7 @@ spell_modifier_cb (const void *pointer, void *data,
     color_error = weechat_color (weechat_config_string (spell_config_color_misspelled));
 
     length = strlen (string);
-    result = weechat_string_dyn_alloc (length * 2);
+    result = weechat_string_dyn_alloc ((length * 2) + 1);
     if (!result)
         return NULL;