sanity check inputs from server sooner

cli/afl-fuzz.cpp

@@ -73,6 +73,12 @@ int32_t AflFuzz_t::getAllInputs()
 			return 1;
 		}
 
+		if(yaml["files"].size()!=1)
+		{
+			cout <<"Cannot flatten files of: " << endl;
+			cout << yaml << endl;
+			return 1;
+		}
 		auto input_files = FlatInput_t();
 		for(auto file : yaml["files"])
 		{