bug#26373: [PATCH 5/6] import cran: Ensure substring indices are valid.

[Ricardo Wurmus]

* guix/import/cran.scm (package->upstream-name): Check that "start" and "end"
are valid before using them as substring indices.
---
 guix/import/cran.scm | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/guix/import/cran.scm b/guix/import/cran.scm
index 423835637..557d694ad 100644
--- a/guix/import/cran.scm
+++ b/guix/import/cran.scm
@@ -384,9 +384,10 @@ dependencies."
              ((or (? string? url) (url _ ...))
               (let ((end   (string-rindex url #\_))
                     (start (string-rindex url #\/)))
-                ;; The URL ends on
-                ;; (string-append "/" name "_" version ".tar.gz")
-                (substring url (+ start 1) end)))
+                (and start end
+                     ;; The URL ends on
+                     ;; (string-append "/" name "_" version ".tar.gz")
+                     (substring url (+ start 1) end))))
              (_ #f)))
           (_ #f)))))
 
-- 
2.12.2