fix: execute udf function on string field will core dump if length greater than 2048

[dl239]

expand MAX_ALLOC_SIZE to 2M

[github-actions]

HybridSE Linux Test Report

       65 files       231 suites   7m 9s ⏱️
19 119 tests 19 119 ✔️ 0 💤 0 ❌

Results for commit ad6c3c7.

[github-actions]

HybridSE Mac Test Report

       65 files       231 suites   6m 57s ⏱️
19 119 tests 19 119 ✔️ 0 💤 0 ❌

Results for commit ad6c3c7.

[codecov]

Codecov Report

Merging #1540 (ad6c3c7) into branch-0.4 (cc13b9f) will increase coverage by 0.06%.
The diff coverage is 85.82%.

@@               Coverage Diff                @@
##             branch-0.4    #1540      +/-   ##
================================================
+ Coverage         65.46%   65.52%   +0.06%     
  Complexity          222      222              
================================================
  Files               570      570              
  Lines            106386   106490     +104     
  Branches            854      835      -19     
================================================
+ Hits              69642    69781     +139     
+ Misses            36598    36563      -35     
  Partials            146      146              

Impacted Files	Coverage Δ
...m/openmldb/taskmanager/spark/SparkJobManager.scala	0.00% <0.00%> (ø)
src/cmd/openmldb.cc	0.00% <ø> (ø)
src/tablet/tablet_impl.h	100.00% <ø> (ø)
hybridse/src/udf/containers.h	90.90% <25.00%> (-4.26%)	⬇️
hybridse/src/udf/default_defs/feature_zero_def.cc	82.69% <25.00%> (-1.44%)	⬇️
src/cmd/sql_cmd.h	21.12% <50.00%> (-0.30%)	⬇️
hybridse/src/udf/udf.cc	84.74% <60.00%> (-0.22%)	⬇️
..._4paradigm/openmldb/batch/nodes/LoadDataPlan.scala	61.11% <85.71%> (+0.73%)	⬆️
hybridse/src/codegen/udf_ir_builder_test.cc	99.44% <100.00%> (+0.01%)	⬆️
hybridse/src/udf/default_udf_library.cc	96.31% <100.00%> (+0.07%)	⬆️
... and 15 more

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[github-actions]

Linux Test Report

     102 files       228 suites   43m 15s ⏱️
  8 593 tests   8 590 ✔️ 3 💤 0 ❌
12 715 runs  12 712 ✔️ 3 💤 0 ❌

Results for commit ad6c3c7.

[tobegit3hub]

LGTM