fuzztest: Better coverage of static data in oneof (#647)

Previously added static_message member of the oneof
had the shortcoming that the first member of the submessage
was also a pointer. So when it aliased with a pointer to a message,
it was still a valid NULL pointer.
diff --git a/tests/alltypes_pointer/alltypes.options b/tests/alltypes_pointer/alltypes.options
index fe0e3a4..9f58aff 100644
--- a/tests/alltypes_pointer/alltypes.options
+++ b/tests/alltypes_pointer/alltypes.options
@@ -1,6 +1,7 @@
 # Generate all fields as pointers.
 * type:FT_POINTER
 *.static_msg type:FT_STATIC
+SubMessage.substuff1 type:FT_STATIC max_size:8
 *.*fbytes fixed_length:true max_size:4
 *.*farray fixed_count:true max_count:5
 *.*farray2 fixed_count:true max_count:3