[gnumeric] Protect against crash of 637142

[Andreas J. Guelzow <guelzow src gnome org>]

commit dd0e5b2f0aace7d7ea04b19a7e87644456bdf533
Author: Andreas J Guelzow <aguelzow pyrshep ca>
Date:   Sun Dec 26 16:31:29 2010 -0700

    Protect against crash of 637142
    
    2010-12-26  Andreas J. Guelzow <aguelzow pyrshep ca>
    
    	* src/go-data-cache-field.c (go_data_cache_field_get_val): protect
    	against crashes

 ChangeLog                 |    5 +++++
 src/go-data-cache-field.c |   15 ++++++++++++---
 2 files changed, 17 insertions(+), 3 deletions(-)
---
diff --git a/ChangeLog b/ChangeLog
index 5a09ad6..122096e 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,8 @@
+2010-12-26  Andreas J. Guelzow <aguelzow pyrshep ca>
+
+	* src/go-data-cache-field.c (go_data_cache_field_get_val): protect 
+	against crashes
+	
 2010-12-26  Jean Brefort  <jean brefort normalesup org>
 
 	* src/sheet-control-gui.c (scg_scale_changed): reposition sheet objects to
diff --git a/src/go-data-cache-field.c b/src/go-data-cache-field.c
index 62ed573..2b0d6d9 100644
--- a/src/go-data-cache-field.c
+++ b/src/go-data-cache-field.c
@@ -215,9 +215,18 @@ go_data_cache_field_get_val (GODataCacheField const *field, unsigned int record_
 	switch (field->ref_type) {
 	case GO_DATA_CACHE_FIELD_TYPE_NONE   : return NULL;
 	case GO_DATA_CACHE_FIELD_TYPE_INLINE : return *((GOVal **)p);
-	case GO_DATA_CACHE_FIELD_TYPE_INDEXED_I8  : idx = *(guint8 *)p; break;
-	case GO_DATA_CACHE_FIELD_TYPE_INDEXED_I16 : idx = *(guint16 *)p; break;
-	case GO_DATA_CACHE_FIELD_TYPE_INDEXED_I32 : idx = *(guint32 *)p; break;
+	case GO_DATA_CACHE_FIELD_TYPE_INDEXED_I8  : 
+		g_return_val_if_fail (field->offset > 0, NULL);
+		idx = *(guint8 *)p; 
+		break;
+	case GO_DATA_CACHE_FIELD_TYPE_INDEXED_I16 : 
+		g_return_val_if_fail (field->offset > 0, NULL);
+		idx = *(guint16 *)p; 
+		break;
+	case GO_DATA_CACHE_FIELD_TYPE_INDEXED_I32 : 
+		g_return_val_if_fail (field->offset > 0, NULL);
+		idx = *(guint32 *)p; 
+		break;
 	default :
 		g_warning ("unknown field type %d", field->ref_type);
 		return NULL;