fix(modbus): handle malloc/pthread_create errors in set_skip_timer

[hxy7yx]

#2899

[codecov-commenter]

Codecov Report

❌ Patch coverage is 37.50000% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 64.87%. Comparing base (26d8505) to head (e0f8208).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
plugins/modbus/modbus_req.c	37.50%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2901      +/-   ##
==========================================
- Coverage   64.90%   64.87%   -0.04%     
==========================================
  Files         133      133              
  Lines       32463    32470       +7     
==========================================
- Hits        21070    21064       -6     
- Misses      11393    11406      +13     

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