Crash if signing server down

[tomt1664]

INFO: 2024/02/08 12:51:18 attestsigner_http.go:92: Error sending request: Post "http://localhost:8000": EOF
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x8fd9c5]

goroutine 37 [running]:
mainstay/attestation.AttestSignerHttp.GetSigs({{{0x0, 0x0}, 0x0, {0x0, 0x0}, 0x0}, {0xc00002e5a0, 0x15}}, {0xc000a78060, 0x1, ...}, ...)
/root/go/src/mainstay/attestation/attestsigner_http.go:96 +0x545
mainstay/attestation.(*AttestService).doStateNewAttestation(0xc00018f260)
/root/go/src/mainstay/attestation/attestservice.go:457 +0xd90
mainstay/attestation.(*AttestService).doAttestation(0xc00036bf90?)
/root/go/src/mainstay/attestation/attestservice.go:663 +0x53
mainstay/attestation.(*AttestService).Run(0xc00018f260)
/root/go/src/mainstay/attestation/attestservice.go:159 +0x13f
created by main.main
/root/go/src/mainstay/main.go:100 +0x39c