Fix for upcoming work overhauling unsigned-byte arrays

Java bytes are signed, which means that accessing such a value from
Lisp should be converted into a negative value between [-127 0) for
bytes with a the high bit set [128 255).  The in-progress changes to
use unsigned-byte currently inadvertantly "fixes" direct use of such
values to set places in UNSIGNED-BYTE arrays.

<armedbear/abcl#625>

Author: easye

swank/abcl.lisp

@@ -272,7 +272,8 @@
          (octets (make-array len :element-type '(unsigned-byte 8))))
     (loop for i from 0 below len
           for jbyte = (java:jarray-ref jbytes i)
-          do (setf (aref octets i) jbyte))
+          do (setf (aref octets i)
+                   (logand jbyte #xff)))
     octets))
 
 ;;;; External formats