Possible string overflow

[K2IE]

When compiling under Raspberry Pi OS Buster, I received the following warning:

    inlined from ‘aprsis_main’ at aprsis.c:826:3,
    inlined from ‘aprsis_runthread’ at aprsis.c:929:2:
aprsis.c:459:4: warning: ‘memcpy’ specified size between 2147483648 and 4294967295 exceeds maximum object size 2147483647 [-Wstringop-overflow=]
    memcpy(A->rdbuf, A->rdbuf + A->rdbuf_cur,
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      A->rdbuf_len - A->rdbuf_cur);
      ~~~~~~~~~~~~~~~~~~~~~~~~~~~~