packages: Gracefully report packages not found.

Fixes a thinko introduced in 1b846da8c372bee78851439fd9e72b2499115e5a
that would lead to a backtrace when looking for an unknown package.

* gnu/packages.scm (%find-package): Correct logic when checking for
FALLBACK?.

1 files changed, 9 insertions, 0 deletions

diff --git a/tests/packages.scm b/tests/packages.scm
index 9a2dbee35a..823ede1f25 100644
--- a/tests/packages.scm
+++ b/tests/packages.scm
@@ -788,6 +788,15 @@
                       (guix-package "-p" (derivation->output-path prof)
                                     "--search-paths"))))))
 
+(test-equal "specification->package when not found"
+  'quit
+  (catch 'quit
+    (lambda ()
+      ;; This should call 'leave', producing an error message.
+      (specification->package "this-package-does-not-exist"))
+    (lambda (key . args)
+      key)))
+
 (test-end "packages")