Turn throw into assert in a can-not-happen situation

We should not try to instantiate this class if we don't have a file
name.

Author: joto

src/node-persistent-cache.cpp

@@ -4,6 +4,8 @@
 #include "node-persistent-cache.hpp"
 #include "options.hpp"
 
+#include <cassert>
+
 void node_persistent_cache::set(osmid_t id, osmium::Location location)
 {
     m_index->set(static_cast<osmium::unsigned_object_id_type>(id), location);
@@ -17,10 +19,7 @@ osmium::Location node_persistent_cache::get(osmid_t id) const noexcept
 
 node_persistent_cache::node_persistent_cache(const options_t *options)
 {
-    if (options->flat_node_file.empty()) {
-        throw std::runtime_error{"Unable to set up persistent cache: the name "
-                                 "of the flat node file was not set."};
-    }
+    assert(!options->flat_node_file.empty());
 
     m_fname = options->flat_node_file.c_str();
     m_remove_file = options->droptemp;