[gnome-shell] st-entry: don't assume a cursor func has been set

[Cosimo Cecchi <cosimoc src gnome org>]

commit a256a3577952734cd3154c842cb627fe043e1155
Author: Cosimo Cecchi <cosimoc gnome org>
Date:   Tue Jun 6 14:27:18 2017 -0700

    st-entry: don't assume a cursor func has been set
    
    That won't be the case when called from tests.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=783484

 src/st/st-entry.c |    3 ++-
 1 files changed, 2 insertions(+), 1 deletions(-)
---
diff --git a/src/st/st-entry.c b/src/st/st-entry.c
index 152f3eb..d81463d 100644
--- a/src/st/st-entry.c
+++ b/src/st/st-entry.c
@@ -739,7 +739,8 @@ static void
 st_entry_set_cursor (StEntry  *entry,
                      gboolean  use_ibeam)
 {
-  cursor_func (entry, use_ibeam, cursor_func_data);
+  if (cursor_func)
+    cursor_func (entry, use_ibeam, cursor_func_data);
 
   ((StEntryPrivate *)ST_ENTRY_PRIV (entry))->has_ibeam = use_ibeam;
 }