Check the encoded error in the iio structures for utils and examples

[dNechita]

PR Description

Replace old way of checking pointers returned by iio_create_context().
The pointers returned by iio_create_context() need to be checked with iio_err().

PR Type

• Bug fix (a change that fixes an issue)
• New feature (a change that adds new functionality)
• Breaking change (a change that affects other repos or cause CIs to fail)

PR Checklist

• I have conducted a self-review of my own code changes
• I have commented new code, particulary complex or unclear areas
• I have checked that I did not intoduced new warnings or errors (CI output)
• I have checked that components that use libiio did not get broken
• I have updated the documentation accordingly (GitHub Pages, READMEs, etc)

[rgetz]

if (!ctx) shows up in alot of the other utils - you should try to update everything at the same time...

[dNechita]

if (!ctx) shows up in alot of the other utils - you should try to update everything at the same time...

This indeed shows up in alot of other utils but some 'ctx' variables do not come directly from the libiio API (e.g. handle_common_opts() ) and are already checked for an error. I did inspect where each 'ctx' variable comes from and updated things where needed.

[dNechita]

V2:

• updated code from utils/ and /examples in order to check for encoded error in several iio structures.