python: check file pointer when loading trusted setup

[jtraglia]

Fixes: #400

Before:

>>> import ckzg
>>> ckzg.load_trusted_setup("sfsfdsfdsfdsf.json")
[1]    60039 segmentation fault  python

After:

>>> import ckzg
>>> ckzg.load_trusted_setup("sfsfdsfdsfdsf.json")
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
RuntimeError: error reading trusted setup

[asn-d6]

Seems like the convention in the other bindings is to do something like:

	fp := C.fopen(cTrustedSetupFile, cMode)
	if fp == nil {
		panic("error reading trusted setup")
	}
	ret := C.load_trusted_setup_file(&settings, fp)
	C.fclose(fp)
	if ret == C.C_KZG_OK {
		loaded = true
	}

where we defer the retval check of load_trusted_setup_file() so that we can unconditionally fclose() once. Should we also adapt this code to that convention?

I think we are also missing an fclose on the error path of the Rust bindings. I can make a new issue for this or handle it here.

[jtraglia]

Yes, good suggestion. I will make this change. And yes to a new PR for the Rust bindings.

[asn-d6]

Pushed at 40f6376

[asn-d6]

Opened #402 for the Rust bindings.