This assert already has hard checks - https://cs.chromium.org/chromium/src/third_party/icu/source/i18n/smpdtfmt.cpp?rcl=dfa798fe694702b43a3debc3290761f22b1acaf8&l=1239, removing security tags.

Looks like a general ICU issue, that exists for quite some time.
jsin, can you take a look please?

Automatically applying components based on information from OWNERS files. If this seems incorrect, please apply the Test-Predator-Wrong-Components label.

ClusterFuzz has detected this issue as fixed in range 48565:48566.

Detailed report: https://clusterfuzz.com/testcase?key=5973332683128832

Fuzzer: inferno_js_fuzzer_c
Job Type: linux_asan_d8_v8_arm_dbg
Platform Id: linux

Crash Type: ASSERT
Crash Address: 
Crash State:
  0 <= value && value < symbolsCount
  icu_59::SimpleDateFormat::subFormat
  icu_59::SimpleDateFormat::_format
  
Sanitizer: address (ASAN)

Regressed: https://clusterfuzz.com/revisions?job=linux_asan_d8_v8_arm_dbg&range=39415:39416
Fixed: https://clusterfuzz.com/revisions?job=linux_asan_d8_v8_arm_dbg&range=48565:48566

Reproducer Testcase: https://clusterfuzz.com/download?testcase_id=5973332683128832

See https://github.com/google/clusterfuzz-tools for more information.

If you suspect that the result above is incorrect, try re-doing that job on the test case report page.

ClusterFuzz testcase 5973332683128832 is verified as fixed, so closing issue as verified.

If this is incorrect, please add ClusterFuzz-Wrong label and re-open the issue.