[Fix] buffer overflow in rspamc counters

[Fix] buffer overflow in rspamc counters

If request to /counters returns no symbols then max_len would have
a negative value:

Results for command: counters (0.003 seconds)
=================================================================
==22096==ERROR: AddressSanitizer: negative-size-param: (size=-2147483604)
    #0 0x33ff13 in __asan_memset (/usr/bin/rspamc+0x33ff13)
    #1 0x383432 in rspamc_counters_output /usr/src/debug/rspamd/src/client/rspamc.c:1064:2
    #2 0x388c49 in rspamc_client_cb /usr/src/debug/rspamd/src/client/rspamc.c:1600:6
    ...