panic when loading volumes concurrently

[qieqieplus]

Describe the bug
Loading same volume more than once leads to SIGSEGV

System Setup

• List the command line to start "weed master", "weed volume", "weed filer", "weed s3", "weed mount".
• OS version
• output of weed version
• if using filer, show the content of filer.toml

1. start server: ./weed server

2. upload some random files

3. restart with "-volume.index leveldb"
   ./weed server -volume.index leveldb

4. crash log

I1216 18:24:45 91019 master_grpc_server.go:250] + client master@10.170.160.252:56816
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/2.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/4.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/3.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/2.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/3.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/1.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/1.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/4.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/5.ldb
I1216 18:24:45 91019 volume_loading.go:128] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/5.ldb
I1216 18:24:45 91019 volume_loading.go:135] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/4.ldb error: resource temporarily unavailable
I1216 18:24:45 91019 volume_loading.go:135] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/2.ldb error: resource temporarily unavailable
I1216 18:24:45 91019 volume_loading.go:135] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/3.ldb error: resource temporarily unavailable
I1216 18:24:45 91019 volume_loading.go:135] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/1.ldb error: resource temporarily unavailable
I1216 18:24:45 91019 volume_loading.go:135] loading leveldb /var/folders/1x/lbz9zhkd1_n0vd2zlmsl_0n40000gn/T/5.ldb error: resource temporarily unavailable
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x188 pc=0x49e80be]

Expected behavior
not crash

[chrislusf]

Added a fix to address this properly.

[qieqieplus]

These two fixes are basically doing the same thing in different places.