another garbage collector bugfix

src/basic/engine/varstr_garbage_collect.s

@@ -307,7 +307,16 @@ varstr_garbage_collect_unused:
 #endif
 
 	ldy #$00
-	lda (TXTPTR), y
+
+#if CONFIG_MEMORY_MODEL_60K
+	ldx #<TXTPTR
+	jsr peek_under_roms
+#elif CONFIG_MEMORY_MODEL_46K || CONFIG_MEMORY_MODEL_50K
+	jsr peek_under_roms_via_TXTPTR
+#else // CONFIG_MEMORY_MODEL_38K
+	lda (TXTPTR),y
+#endif
+
 	tax
 
 	// Increase INDEX (shift offset) by string length + back-pointer length