_type: exit if input is not available

Currently when

  BUF _type

is executed, if there is a newline immediately after _type, no input is
available. C< returns 0, and it is stored in the buffer.  _type then
proceeds to read until another newline is reached, and the resulting
buffer contents looks like "\x00xyz\x0d" (in C string notation).

This change makes _type exit early in this case and keep the buffer
unchanged.

_type is used by editor words such as I and F, so this bug affects them
as well.

blk/108

@@ -1,4 +1,5 @@
 : _type ( buf -- )
+    C<? NOT IF DROP EXIT THEN
     C< DUP 0xd = IF 2DROP EXIT THEN SWAP DUP _zbuf ( c a )
     BEGIN ( c a ) C!+ C< TUCK 0x0d = UNTIL ( c a ) C! ;
 ( user-facing lines are 1-based )
@@ -13,4 +14,3 @@
 
 
 
-