Windows crashes with netmgr-based statschannel

The following crash happened in the statistics system test for both Windows builds (Release and Debug) in a pipeline run for f27c0c32:

16-Jul-2020 21:34:32.694 c:\builds\isc-projects\bind9\lib\isc\buffer.c:76: REQUIRE((((b) != ((void *)0)) && (((const isc__magic_t *)(b))->magic == (0x42756621U)))) failed

Given that:

• I have not seen such crashes before,

• a pipeline ran for d8e6b32a did not trigger these crashes,

• the only code-related MR merged between the last successful pipeline and the first crashing one was !3847 (merged),

there is a fair chance that !3847 (merged) introduced a Windows-specific bug (or a generic one that is simply easier to trigger on Windows).

@each: assigning you as the author for !3847 (merged), please take a look.