wal: fix panic when decoder not set

[spzala]

The Create function doc says a WAL is ready to read right after but it results in a panic. A returned WAL from call to Open is ready to read which is correctly mentioned in the Open func.

[spzala]

/cc @xiang90 @gyuho @mitake

[codecov-io]

Codecov Report

Merging #11807 into master will decrease coverage by 0.50%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master   #11807      +/-   ##
==========================================
- Coverage   66.50%   65.99%   -0.51%     
==========================================
  Files         403      403              
  Lines       36981    36983       +2     
==========================================
- Hits        24595    24408     -187     
- Misses      10888    11070     +182     
- Partials     1498     1505       +7     

Impacted Files	Coverage Δ
wal/wal.go	56.96% <100.00%> (+0.18%)	⬆️
client/members.go	65.32% <0.00%> (-20.17%)	⬇️
auth/store.go	52.38% <0.00%> (-19.25%)	⬇️
client/keys.go	78.39% <0.00%> (-13.07%)	⬇️
proxy/grpcproxy/register.go	72.50% <0.00%> (-10.00%)	⬇️
pkg/fileutil/purge.go	69.56% <0.00%> (-6.53%)	⬇️
clientv3/concurrency/mutex.go	61.64% <0.00%> (-5.48%)	⬇️
raft/tracker/inflights.go	91.83% <0.00%> (-4.09%)	⬇️
pkg/testutil/recorder.go	77.77% <0.00%> (-3.71%)	⬇️
etcdserver/api/v3rpc/member.go	87.09% <0.00%> (-3.23%)	⬇️
... and 17 more

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[spzala]

Thanks @gyuho