fix(modbus):clear recv buf before sending

[hxy7yx]

No description provided.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 80.00000% with 4 lines in your changes missing coverage. Please review.

Project coverage is 73.82%. Comparing base (7dad962) to head (d33fff5).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
src/connection/connection.c	78.94%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2344      +/-   ##
==========================================
- Coverage   74.63%   73.82%   -0.82%     
==========================================
  Files         116      116              
  Lines       23960    24261     +301     
==========================================
+ Hits        17883    17911      +28     
- Misses       6077     6350     +273     

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