search: Fix data race in searchSymbolsInRepo()
Created by: tsenart
Fixes #5006
Read at 0x00c000940320 by goroutine 104:
github.com/sourcegraph/sourcegraph/cmd/frontend/graphqlbackend.searchSymbolsInRepo()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/graphqlbackend/search_symbols.go:157 +0x303
github.com/sourcegraph/sourcegraph/cmd/frontend/graphqlbackend.searchSymbols.func2()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/graphqlbackend/search_symbols.go:81 +0x1a2
github.com/sourcegraph/sourcegraph/cmd/frontend/internal/goroutine.Go.func1()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/internal/goroutine/goroutine.go:24 +0x50
Previous write at 0x00c000940320 by goroutine 108:
github.com/sourcegraph/sourcegraph/cmd/frontend/graphqlbackend.zoektIndexedRepos()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/graphqlbackend/textsearch.go:863 +0x634
github.com/sourcegraph/sourcegraph/cmd/frontend/graphqlbackend.searchFilesInRepos()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/graphqlbackend/textsearch.go:899 +0x2316
github.com/sourcegraph/sourcegraph/cmd/frontend/graphqlbackend.(*searchResolver).doResults.func5()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/graphqlbackend/search_results.go:922 +0x12e
github.com/sourcegraph/sourcegraph/cmd/frontend/internal/goroutine.Go.func1()
/Users/issactrotts/sg/sourcegraph/cmd/frontend/internal/goroutine/goroutine.go:24 +0x50
Test plan: