fix: do not exit go-routine upon ctx.Done() on receiver side

[harshavardhana]

If there are concurrent callers sharing a context, it may
happen that while the "reqCh" gets a "send" attempt, context
might be closed and returning.

[harshavardhana]

This fixes a bug

panic: send on closed channel

goroutine 797459 [running]:
github.com/minio/minio-go/v7.(*Object).doGetRequest(_, {{0xc01a2dc000, 0x2000, 0x2000}, 0xba2000, 0x0, 0x1, 0x0, 0x1, 0x0, ...})
        github.com/minio/minio-go/v7@v7.0.31/api-get-object.go:319 +0xdc
github.com/minio/minio-go/v7.(*Object).Read(0xc06c571180, {0xc01a2dc000?, 0x1c0?, 0xc004002400?})
        github.com/minio/minio-go/v7@v7.0.31/api-get-object.go:399 +0x248
io.discard.ReadFrom({}, {0x4cd0e20, 0xc06c571180})
        io/io.go:610 +0x72
io.copyBuffer({0x4cd7d80, 0x6023d20}, {0x4cd0e20, 0xc06c571180}, {0x0, 0x0, 0x0})
        io/io.go:412 +0x14b
io.Copy(...)
        io/io.go:385
github.com/minio/minio/cmd.selfSpeedTest.func4(0x8)
        github.com/minio/minio/cmd/perf-tests.go:149 +0x3f5
created by github.com/minio/minio/cmd.selfSpeedTest
        github.com/minio/minio/cmd/perf-tests.go:124 +0x8d6

[vadmeste]

LGTM