able to set autoload entries even the parameters are not correct

[hchen59]

Description:
=========
Able to set autoload entries with incorrect sink/source/module name, via API pa_context_add_autoload().

Details:
======
1> Try to add autoload sink: pa_context_add_autoload(context, sink_name, PA_AUTOLOAD_SINK, module_name, module_argv, context_index_callback, NULL) => use sink_name="fakesink", module_name="fakemodule", module_argv=NULL. Return index=0

2> Try to add autoload source: pa_context_add_autoload (context, source_name, PA_AUTOLOAD_SOURCE, module_name, module_argv, context_index_callback, NULL) =>use source_name="fakesink", module_name="fakemodule", module_argv=NULL. Return index=1

Go to list-autoload in pacmd, both upper two entries are listed. This behavior seems incorrect. At least PA should check specified module name according to module list supported.

[coling]

Dupe of #417 for reference sake :)