small optimization in string handling code

src/basic/engine_variables/tmpstr_free.s

@@ -64,7 +64,7 @@ tmpstr_free_loop:
 
 	sty DSCPNT+0
 	phx_trash_a
-	jsr varstr_free_non_0
+	jsr varstr_free_no_checks
 	plx_trash_a
 
 	// Mark empty string as free and quit

src/basic/engine_variables/tmpstr_free_all.s

@@ -28,7 +28,7 @@ tmpstr_free_all_loop:
 	sta DSCPNT+2
 
 	phx_trash_a
-	jsr varstr_free_non_0
+	jsr varstr_free_no_checks
 	plx_trash_a
 !:
 	inx

src/basic/engine_variables/varstr_free.s

@@ -16,21 +16,20 @@ varstr_free:
 	// Check the string size - do not do anything if 0
 
 	lda DSCPNT+0
-	bne varstr_free_non_0
+	bne !+
 	rts
-
-varstr_free_non_0: // entry point to be used when it is clear that string is not empty
-
+!:
 	// Check if string is above FRETOP (in string area)
-	// XXX move this check out of the routine
 
 	jsr helper_cmp_fretop
-	bcs !+
+	bcs varstr_free_no_checks
 
 	// No, it does not belong to string area - quit
 
 	rts
-!:
+
+varstr_free_no_checks: // entry point to be used when it is clear that string is not empty and above FRETOP
+
 	// Check if this is the lowest string
 
 	bne varstr_free_inside