Shrink diff in two places

Stuff crept in there.

src/libstore/content-address.cc

@@ -17,8 +17,9 @@ std::string makeFileIngestionPrefix(FileIngestionMethod m)
         return "";
     case FileIngestionMethod::Recursive:
         return "r:";
+    default:
+        throw Error("impossible, caught both cases");
     }
-    assert(false);
 }
 
 std::string makeFixedOutputCA(FileIngestionMethod method, const Hash & hash)

src/libstore/make-content-addressed.cc

@@ -31,12 +31,14 @@ std::map<StorePath, StorePath> makeContentAddressed(
         for (auto & ref : oldInfo->references) {
             if (ref == path)
                 refs.self = true;
-            auto i = remappings.find(ref);
-            auto replacement = i != remappings.end() ? i->second : ref;
-            // FIXME: warn about unremapped paths?
-            if (replacement != ref) {
-                rewrites.insert_or_assign(srcStore.printStorePath(ref), srcStore.printStorePath(replacement));
-                refs.others.insert(std::move(replacement));
+            else {
+                auto i = remappings.find(ref);
+                auto replacement = i != remappings.end() ? i->second : ref;
+                // FIXME: warn about unremapped paths?
+                if (replacement != ref) {
+                    rewrites.insert_or_assign(srcStore.printStorePath(ref), srcStore.printStorePath(replacement));
+                    refs.others.insert(std::move(replacement));
+                }
             }
         }