hover.contents can be empty

[fstanis]

From #835: it seems that some users (myself included) sometimes get TypeError: Cannot read property 'value' of undefined at SignatureHelpProvider.hoverToSignatureHelp which may or may not be causing IntelliSense to fail.

Seems the culprit is on this line:

https://github.com/rust-lang/vscode-rust/blob/c957ad5ff0eee266e9b2506ad3367fcef1f17cab/src/providers/signatureHelpProvider.ts#L114

The only way that'd be caused is if hover.contents is an empty array (in which case hover.contents[0] returns undefined and hover.contents[0].value throws). I'm not sure if this is an expected behavior for hover? If so, sounds like an easy fix to add an array length check.