Don't panic when an external crate can't be resolved

This isn't actually a bug, it can occur when rustdoc tries to resolve a
crate that isn't used in the main code.

src/test/rustdoc-ui/intra-doc/unused-extern-crate.rs

@@ -0,0 +1,5 @@
+// compile-flags: --extern zip=whatever.rlib
+#![deny(broken_intra_doc_links)]
+/// See [zip] crate.
+//~^ ERROR unresolved
+pub struct ArrayZip;

src/test/rustdoc-ui/intra-doc/unused-extern-crate.stderr

@@ -0,0 +1,15 @@
+error: unresolved link to `zip`
+  --> $DIR/unused-extern-crate.rs:3:10
+   |
+LL | /// See [zip] crate.
+   |          ^^^ no item named `zip` in scope
+   |
+note: the lint level is defined here
+  --> $DIR/unused-extern-crate.rs:2:9
+   |
+LL | #![deny(broken_intra_doc_links)]
+   |         ^^^^^^^^^^^^^^^^^^^^^^
+   = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
+
+error: aborting due to previous error
+