Work around uninitialized warning in cc_kcm.c

Some versions of clang erroneously detect use of an uninitialized
variable reply_len in kcmio_call() when building on non-Mac platforms.
Initialize it to work around this warning.

(cherry picked from commit 40b007c)

ticket: 8335
version_fixed: 1.13.4
tags: -pullup
status: resolved

src/lib/krb5/ccache/cc_kcm.c

@@ -377,7 +377,7 @@ static krb5_error_code
 kcmio_call(krb5_context context, struct kcmio *io, struct kcmreq *req)
 {
     krb5_error_code ret;
-    size_t reply_len;
+    size_t reply_len = 0;
 
     if (k5_buf_status(&req->reqbuf) != 0)
         return ENOMEM;