Linux+SITL: Fix risk of wrong prng return

[rsaxvc]

Summary

Return false from get_random_vals() if it fails to fully populate the destination buffer.

Classification & Testing (check all that apply and add your own)

• Checked by a human programmer
• Non-functional change
• No-binary change
• Infrastructure change (e.g. unit tests, helper scripts)
• Automated test(s) verify changes (e.g. unit test, autotest)
• Tested manually, description below (e.g. SITL)
• Tested on hardware
• Logs attached
• Logs available on request

Testing steps:

1. instrumented existing implementation
2. used existing implementation to read random numbers into a 1MB array in a loop with a no-op SIGINT handler while mashing control-C
3. Confirmed signals can cause read() to return short reads(positive return code but less than specified size), but get_random_vals() returned true.
4. Fixed it.
5. Verified return code is now false under the above signal interruption.

Description

Fix a footgun in get_random_vals() when read() returns early due to a signal handler(as may happen during ArduPilot shutdown on SITL or Linux). Some systems will read blocks of 4096B at a time, though the manual states early returns may occur as small as 256B. I couldn't find any mainline ArduPilot code that read()s a large enough buffer to trigger this.