Replace assert of buffer block SPIRType with exception in get_buffer_block_flags

[chyyran]

The assert in ParsedIR::get_buffer_block_flags is exposed to user code if Compiler::get_buffer_block_flags or spvc_compiler_get_buffer_block_decorations is called with a non-buffer variable (handle).

Particularly since the C API has no way of getting a SPIRType given only a spvc_variable_id this should not cause an assertion fail or be ignored but throw an exception.