internal/lsp/source: avoid panic in HoverIdentifier
internal/lsp/source: avoid panic in HoverIdentifier

Fixes golang/go#52211

Change-Id: I2ab5dbba4bb06092a234269d1ee65493f60fc6fc
Reviewed-on: https://go-review.googlesource.com/c/tools/+/399622
Run-TryBot: Robert Findley <9179eee875d1ee628c37fb47d7e749e855ffd21f@google.com>
gopls-CI: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Hyang-Ah Hana Kim <d7e82c3c07f92613bcbf9a1e88f6b96c13caa3b0@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/tools,golang/tools,golang/tools,golang/tools,golang/tools