fix, null pointer exception in data-transfer-handler

[zl03jsj]

Since GetDeal returns an error,

https://github.com/filecoin-project/venus-market/blob/4d8533af0138c2d003dade56ced721ae1eac1b23/retrievalprovider/datatransfer_handler.go#L42-L50

The first returned param deal would be nil,
Should not use it again, just returns the error is ok.

[codecov-commenter]

Codecov Report

Merging #136 (973771a) into main (4d8533a) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main     #136      +/-   ##
==========================================
+ Coverage   18.75%   18.78%   +0.02%     
==========================================
  Files         169      169              
  Lines       17742    17735       -7     
==========================================
+ Hits         3328     3331       +3     
+ Misses      13980    13971       -9     
+ Partials      434      433       -1     

Flag	Coverage Δ
unittests	18.78% <0.00%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
retrievalprovider/datatransfer_handler.go	0.00% <0.00%> (ø)
blockstore/badger/blockstore.go	56.36% <0.00%> (+0.45%)	⬆️