ensure clang is not confused by attribute value conversion code (assert silences warning)

attributes/maestro-schema.c

@@ -730,10 +730,12 @@ mstro_attribute_val__compute_size(enum mstro_stp_val_kind kind,
       *val_size = sizeof(double); break;
     case MSTRO_STP_STR:
     case MSTRO_STP_REGEX: 
-      if(string)
+      if(string!=NULL)
         *val_size = strlen(string)+1;
-      else
+      else {
+	assert(val!=NULL);
         *val_size = strlen((char*)val)+1;
+      }
       break;
     case MSTRO_STP_TIMESTAMP:
       *val_size = sizeof(mstro_timestamp);