[libgweather] GWeatherTimezone: Add guard to gweather_timezone_get_by_tzid()

[Bastien Nocera <hadess src gnome org>]

commit 10f1bf6339ea9f6aabc2bb3beeef23f037594384
Author: Bastien Nocera <hadess hadess net>
Date:   Wed Dec 6 14:43:43 2017 +0100

    GWeatherTimezone: Add guard to gweather_timezone_get_by_tzid()
    
    This led to a crash in my badly written test code, a g_warning() is
    more useful to show incorrect API usage.
    
    https://bugzilla.gnome.org/show_bug.cgi?id=791307

 libgweather/gweather-timezone.c |    2 ++
 1 files changed, 2 insertions(+), 0 deletions(-)
---
diff --git a/libgweather/gweather-timezone.c b/libgweather/gweather-timezone.c
index a120061..a513fd9 100644
--- a/libgweather/gweather-timezone.c
+++ b/libgweather/gweather-timezone.c
@@ -165,6 +165,8 @@ gweather_timezone_get_by_tzid (const char *tzid)
 {
     GWeatherLocation *world;
 
+    g_return_val_if_fail (tzid != NULL, NULL);
+
     world = gweather_location_get_world ();
 
     return g_hash_table_lookup (world->timezone_cache, tzid);