Fix: improves stability on get_available_memory

[RuffaloLavoisier]

What:

1. Added safety to check if it is null before using the file pointer.
2. Check buf null pointer, change fread() condition check.

Why:

1. File pointer may be null due to environmental factors.
2. If the fread fails, the buffer may be NULL.

How:

1. Add check condition for file pointer.
2. Add check condition for buf

Checklist:

• Tests
• PR merge commit message adjusted

[jjnicola]

Hi @RuffaloLavoisier ! thanks for your collaboration. Please, take a look at https://github.com/greenbone/openvas-scanner/tree/main/RELICENSE

[RuffaloLavoisier]

@jjnicola

Hi there
When will it be merged into the main branch?