azurerm_application_gateway: fix panic in backend and probe pick hostname validation

[katbyte]

No description provided.

[katbyte]

Fixes:

2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4: panic: runtime error: index out of range
2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4: 
2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4: goroutine 477 [running]:
2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4: github.com/terraform-providers/terraform-provider-azurerm/azurerm.flattenApplicationGatewayAuthenticationCertificates(0xc000566960, 0xc0007af0a0, 0x8, 0x2225ae0, 0xc000889360)
2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4:  /opt/teamcity-agent/work/5d79fe75d4460a2f/src/github.com/terraform-providers/terraform-provider-azurerm/azurerm/resource_arm_application_gateway.go:1143 +0x55b
2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4: github.com/terraform-providers/terraform-provider-azurerm/azurerm.resourceArmApplicationGatewayRead(0xc0007af0a0, 0x2583b20, 0xc0002e0000, 0xc0007af0a0, 0x0)
2019-05-13T16:35:34.203-0400 [DEBUG] plugin.terraform-provider-azurerm_v1.23.0_x4:  /opt/teamcity-agent/work/5d79fe75d4460a2f/src/github.com/terraform-providers/terraform-provider-azurerm/azurerm/resource_arm_application_gateway.go:988 +0x644

[mbfrahry]

LGTM!

[ghost]

This has been released in version 1.28.0 of the provider. Please see the Terraform documentation on provider versioning or reach out if you need any assistance upgrading. As an example:

provider "azurerm" {
	version = "~> 1.28.0"
}
# ... other configuration ...

[ghost]

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