Prevent underflow in memcached_*_response_decrement macros

[adsr]

Hello, we ran into a case where instance->cursor_active_ underflows here:

libmemcached/src/libmemcached/response.cc

Line 748 in bcc1a6e

memcached_server_response_decrement(instance);

leading to a near-infinite loop here:

libmemcached/src/libmemcached/get.cc

Lines 190 to 192 in bcc1a6e

	while (instance->response_count()) {
	(void) memcached_response(instance, buffer, MEMCACHED_DEFAULT_COMMAND_SIZE, &ptr->result);
	}

It seems to occur once every couple hours on a staging host under Apache 2.4 and PHP 7.3 against memcached 1.6.6. We weren't able to pinpoint the exact conditions to reproduce, nor the source of the bug. Maybe a missing memcached_*_response_increment or memory corruption?

This appears to be a regression from 1.0.18 which we were running previously.

The patch mitigates but doesn't fix the bug.

[codecov-commenter]

Codecov Report

Merging #124 (c8c52cf) into v1.x (bcc1a6e) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff          @@
##            v1.x    #124   +/-   ##
=====================================
- Coverage     68%     68%   -0%     
=====================================
  Files        194     194           
  Lines      17479   17479           
=====================================
- Hits       11930   11915   -15     
- Misses      5549    5564   +15     

Impacted Files	Coverage Δ
src/libmemcached/common.h	100% <ø> (ø)
src/libmemcached/stats.cc	72% <0%> (-4%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update bcc1a6e...c8c52cf. Read the comment docs.