small code reorganization for arrays support

src/basic/engine_runtime/check_stack_space.s

@@ -0,0 +1,12 @@
+// #LAYOUT# STD *       #TAKE
+// #LAYOUT# *   BASIC_0 #TAKE
+// #LAYOUT# *   *       #IGNORE
+
+
+check_stack_space:
+
+	tsx
+	cpx #$60                           // XXX is this a safe threshold?
+	bcc_16 do_FORMULA_TOO_COMPLEX_error
+
+	rts

src/basic/engine_runtime/frmevl.s

@@ -323,9 +323,7 @@ FRMEVL_push_operator_address:
 	// Check if there is enough stack space; this could not have been done earlier,
 	// as this is the place where unary operator support calls
 
-	tsx
-	cpx #$60                           // XXX is this a safe threshold?
-	bcc_16 do_FORMULA_TOO_COMPLEX_error
+	jsr check_stack_space
 
 	// Push the operator address to stack, in a form
 	// suitable for RTS