fix(java): fix a bug of assigning results in cur but not COMPREPLY

At the end of the function `_java_packages', `cur` was modified to be
the list of package names with separators `/` being replaced with
periods `.`.  It does not seem to make sense, and the value is
actually not used after the call of `_java_packages'.

Historically, this line was introduced in commit ff01750.  This
seems to be a reuse of another similar line in `_java_classes` where
the result is assigned to the array `COMPREPLY` but not to `cur`.
This makes a more sense.  In fact, the man page of the `javadoc`
command [1] mentions an example of the package specification, where
the package names have the form of `xxx.yyy.zzz` but not
`xxx/yyy/zzz`.

[1] https://docs.oracle.com/javase/9/javadoc/javadoc-command.htm

Here, we assume that the original intent of the line in ff01750 was
to convert the generated package names to have `xxx.yyy.zzz`, i.e.,
wanted to assign the results to the array `COMPREPLY` but not to
`cur`.  This commit implements the original intent.

Author: akinomyoga

completions/java

@@ -128,7 +128,7 @@ _comp_cmd_java__packages()
         # keep only packages with the package suffix `/` being removed
         _comp_split -l COMPREPLY "$(printf '%s\n' "${COMPREPLY[@]}" | command sed -n 's,/$,,p')"
         # convert path syntax to package syntax
-        cur="${COMPREPLY[*]//\//.}"
+        ((${#COMPREPLY[@]})) && COMPREPLY=("${COMPREPLY[@]//\//.}")
     fi
 }