core: fix potential NULL pointer in function gui_color_emphasize

ChangeLog

@@ -13,6 +13,7 @@ http://weechat.org/files/releasenotes/ReleaseNotes-devel.html[release notes]
 
 == Version 0.4.4 (under dev)
 
+* core: fix potential NULL pointer in function gui_color_emphasize
 * core: use same return code and message in all commands when arguments are
   wrong/missing
 * core: add option "-beep" in command /print

src/gui/gui-color.c

@@ -615,7 +615,12 @@ gui_color_emphasize (const char *string,
     int rc, length_search, length_emphasis, length_result;
     int pos1, pos2, real_pos1, real_pos2, count_emphasis;
 
-    if (!string && !regex)
+    /* string is required */
+    if (!string)
+        return NULL;
+
+    /* search or regex is required */
+    if (!search && !regex)
         return NULL;
 
     color_emphasis = gui_color_get_custom ("emphasis");