BLI_args: disallow zero to be used as a pass value

Introduced recently in 09139e41ed.
While this worked in the cases it was used, '--threads' for example
was failing to parse the number when it's pass was set to 0.

Increase the enum values to start at 1 &
add asserts so this wont happen again.

source/creator/creator_intern.h

@@ -57,14 +57,14 @@ extern struct ApplicationState app_state; /* creator.c */
  * Keep in order of execution.
  */
 enum {
-  ARG_PASS_ENVIRONMENT = 0,
-  ARG_PASS_SETTINGS = 1,
+  ARG_PASS_ENVIRONMENT = 1,
+  ARG_PASS_SETTINGS = 2,
   /** Windowing & graphical settings, ignored in background mode. */
-  ARG_PASS_SETTINGS_GUI = 2,
-  ARG_PASS_SETTINGS_FORCE = 3,
+  ARG_PASS_SETTINGS_GUI = 3,
+  ARG_PASS_SETTINGS_FORCE = 4,
 
   /** Actions & fall back to loading blend file. */
-  ARG_PASS_FINAL = 4,
+  ARG_PASS_FINAL = 5,
 };
 
 /* for the callbacks: */