C front-end: pragma conflict checking must handle empty stack

We do not necessarily have any pragmas set, and therefore must be able
to handle an empty stack.

Surprisingly, this only seg faulted when building in exactly the way
check-ubuntu-20_04-make-gcc is doing this in CI, and only on one test
added in the next commit.

Author: tautschnig

src/ansi-c/ansi_c_parser.cpp

@@ -203,6 +203,9 @@ void ansi_c_parsert::pragma_cprover_add_check(
 
 bool ansi_c_parsert::pragma_cprover_clash(const irep_idt &name, bool enabled)
 {
+  if(pragma_cprover_stack.empty())
+    return false;
+
   auto top = pragma_cprover_stack.back();
   auto found = top.find(name);
   return found != top.end() && found->second != enabled;