kernels: Fix std::out_of_range if no kernels to protect

In case we did not find any kernels to protect, the regular expression
will be empty, and trying to substr(1) it will fail.

1 files changed, 6 insertions, 1 deletions

diff --git a/apt-pkg/algorithms.cc b/apt-pkg/algorithms.cc
index 260a8ac41..4c267c91c 100644
--- a/apt-pkg/algorithms.cc
+++ b/apt-pkg/algorithms.cc
@@ -1606,7 +1606,12 @@ std::string GetProtectedKernelsRegex(pkgCache *cache, bool ReturnRemove)
       }
    }
 
-   auto re = ss.str().substr(1);
+   auto re_with_leading_or = ss.str();
+
+   if (re_with_leading_or.empty())
+      return "";
+
+   auto re = re_with_leading_or.substr(1);
    if (Debug)
       std::clog << "Kernel protection regex: " << re << "\n";