x86/microcode/intel: Do the mc_saved_src NULL check first

... and only then deref it. Also, shorten some variable names and rename
others so as to diminish the ubiquitous presence of the "mc_" prefix
everywhere and make it a bit more readable.

Use kcalloc so that we don't kfree() uninitialized memory on the unwind
path, as suggested by Quentin.

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Quentin Casasnovas <quentin.casasnovas@oracle.com>