Add an exception for unresolved syscall numbers

This eases debugging when libseccomp doesn't know the name of a syscall.

Author: otargowski

src/seccomp/SeccompListener.cc

@@ -193,6 +193,10 @@ std::string SeccompListener::resolveSyscallNumber(
     char* name = seccomp_syscall_resolve_num_arch(
             SeccompContext::SECCOMP_FILTER_ARCHITECTURES.at(arch),
             syscallNumber);
+    if (name == NULL)
+        throw Exception(
+                "Can't resolve the name of syscall number " +
+                std::to_string(syscallNumber));
     std::string syscallName(name);
     free(name);
     return syscallName;