Crash if the TLS certificate is valid for a different domain name

[kquinsland]

Steps for Reproduction

1. Set up a reverse proxy that terminates TLS in front of pihole. For example, lets say that the TLS certificate is good for piHole.some-domain.com (that's hole with one e).
2. Launch binary with the following arguments:

            - name: PIHOLE_HOSTNAME
              value: "piHolee.some-domain.com"

Note: The typo is Holee Should be one e

3. Run the exporter
4. Observe in logs:

<...snip...>
 Starting HTTP server
2021/04/15 17:41:33 An error has occured during retrieving PI-Hole statistics Get "https://pihole.some-domain.com:443/admin/api.php?summaryRaw&overTimeData&topItems&recentItems&getQueryTypes&getForwardDestinations&getQuerySources&jsonForceObject&auth=<...>": x509: certificate is valid for pihole.some-domain.com, not piholee.some-domain.com
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x40 pc=0x80d88a]
goroutine 54 [running]:
github.com/eko/pihole-exporter/internal/pihole.(*Client).getStatistics(0xc0002803f0, 0xc00003c790)
	/go/src/github.com/eko/pihole-exporter/internal/pihole/client.go:163 +0x2ea
github.com/eko/pihole-exporter/internal/pihole.(*Client).Scrape(0xc0002803f0)
	/go/src/github.com/eko/pihole-exporter/internal/pihole/client.go:61 +0x88
created by main.initPiHoleClient
	/go/src/github.com/eko/pihole-exporter/main.go:37 +0xc5

Expected behavior:

Ideally, no crash :/

Actual behavior:

See stack trace above

Platforms:

Kubernetes on Ubuntu, 64bit intel.

Versions:

Which versions are you running?

image: "ekofr/pihole-exporter:latest" as of 2021-04-14, so the release would be v0.0.11.