fix multi.Errors panic on nil append

[jeremy-asher]

This extends the nil check for multi.Errors to the Append function to make sure
nil errors do not end up in the slice, causing a panic when the error string is
generated.

I ran into this panic with the QueryBlockByTxID function. The QueryBlock* series of functions lack a nil check before appending errors:

fabric-sdk-go/pkg/fab/channel/ledger.go

Lines 91 to 94 in be7b275

	tprs, errs := queryChaincode(reqCtx, c.chName, cir, targets, verifier)
	responses, errors := getConfigBlocks(tprs)
	errs = multi.Append(errs, errors)

fabric-sdk-go/pkg/fab/channel/ledger.go

Lines 109 to 112 in be7b275

	tprs, errs := queryChaincode(reqCtx, c.chName, cir, targets, verifier)
	responses, errors := getConfigBlocks(tprs)
	errs = multi.Append(errs, errors)

fabric-sdk-go/pkg/fab/channel/ledger.go

Lines 138 to 141 in be7b275

	tprs, errs := queryChaincode(reqCtx, c.chName, cir, targets, verifier)
	responses, errors := getConfigBlocks(tprs)
	errs = multi.Append(errs, errors)

The tests for these functions don't seem to be capable of handling these edge cases, but I added a test for this specific case to the multi test suite. This test fails on the current master (and panics when printing the failure).

If it is preferred, I can just add some nil checks on those functions, but this seemed like a simpler, more universal solution and in the spirit of how the New function works:

fabric-sdk-go/pkg/common/errors/multi/multi.go

Lines 24 to 28 in be7b275

	for _, err := range errs {
	if err != nil {
	errors = append(errors, err)
	}
	}

[codecov]

Codecov Report

Merging #53 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #53      +/-   ##
==========================================
+ Coverage   74.61%   74.66%   +0.05%     
==========================================
  Files         183      183              
  Lines       12788    12790       +2     
==========================================
+ Hits         9542     9550       +8     
+ Misses       2304     2300       -4     
+ Partials      942      940       -2

Impacted Files	Coverage Δ
pkg/common/errors/multi/multi.go	100% <100%> (ø)	⬆️
pkg/util/concurrent/lazyref/lazyref.go	96.31% <0%> (+3.68%)	⬆️

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