[cache] Allow value 0 for face ID.

We never dereference `face_id', and some implementations might use a
running number instead of a pointer.  Additionally, disallowing
value zero was undocumented.

* src/cache/ftccmap.c (FTC_CMapCache_Lookup), src/cache/ftcmanag.c
(FTC_Manager_LookupFace, FTC_Manager_RemoveFaceID): Remove test for
`face_id'.

ChangeLog

@@ -1,3 +1,15 @@
+2016-05-07  Werner Lemberg  <wl@gnu.org>
+
+	[cache] Allow value 0 for face ID.
+
+	We never dereference `face_id', and some implementations might use a
+	running number instead of a pointer.  Additionally, disallowing
+	value zero was undocumented.
+
+	* src/cache/ftccmap.c (FTC_CMapCache_Lookup), src/cache/ftcmanag.c
+	(FTC_Manager_LookupFace, FTC_Manager_RemoveFaceID): Remove test for
+	`face_id'.
+
 2016-05-05  Alexei Podtelezhnikov  <apodtele@gmail.com>
 
 	[smooth] More efficient accounting of conic splits and draws.

src/cache/ftccmap.c

@@ -259,9 +259,6 @@
       return 0;
     }
 
-    if ( !face_id )
-      return 0;
-
     query.face_id    = face_id;
     query.cmap_index = (FT_UInt)cmap_index;
     query.char_code  = char_code;

src/cache/ftcmanag.c

@@ -314,7 +314,7 @@
     FTC_MruNode  mrunode;
 
 
-    if ( !aface || !face_id )
+    if ( !aface )
       return FT_THROW( Invalid_Argument );
 
     *aface = NULL;
@@ -672,7 +672,7 @@
     FT_UInt  nn;
 
 
-    if ( !manager || !face_id )
+    if ( !manager )
       return;
 
     /* this will remove all FTC_SizeNode that correspond to