validate number of indirect desc in pop_avail - #1188

control: job assigned to worker 01KZ4EPDNTRZS6WQ1BYP2HV7B0 [factory aws, i-06d416621ec242152] (queued for 50 s)
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task: starting task 0: "setup"
task: process exited: duration 3121 ms, exit code 0
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task: starting task 1: "rust-toolchain"
task: process exited: duration 18026 ms, exit code 0
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task: starting task 2: "authentication"
task: process exited: duration 15 ms, exit code 0
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task: starting task 3: "clone repository"
task: process exited: duration 1472 ms, exit code 0
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task: starting task 4: "build"
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|    Compiling rustc_version v0.4.1
|    Compiling dirs-sys v0.3.7
|    Compiling nvpair_sys v0.0.0 (/work/oxidecomputer/propolis/crates/nvpair/sys)
|    Compiling dirs v2.0.2
|    Compiling nvpair v0.0.0 (/work/oxidecomputer/propolis/crates/nvpair)
|    Compiling term v0.6.1
|    Compiling viona_api v0.0.0 (/work/oxidecomputer/propolis/crates/viona-api)
|    Compiling syn v3.0.3
|    Compiling serde_derive v1.0.229
|    Compiling garando_pos v0.1.0
|    Compiling garando_errors v0.1.0
|    Compiling garando_syntax v0.1.1
|    Compiling ctest2 v0.4.10
|    Compiling viona_api-hdrchk v0.0.0 (/work/oxidecomputer/propolis/crates/viona-api/header [...]
|     Finished `test` profile [unoptimized + debuginfo] target(s) in 10.70s
|      Running test/main.rs (target/debug/deps/main-fc81d3831646cfa3)
| RUNNING ALL TESTS
| PASSED 257 tests
task: process exited: duration 83334 ms, exit code 0
|W| found 0 output files