strdup and kstrdup: null pointer checks

[SafeCoding233]

The lib func strdup and kstrdup may return null pointers on memory allocation failures.

I added checks for the unchecked calls to those two functions.

Specifically, for the four firmware.c files in different patches, the null pointer is actually checked by the caller function. But it lacks an error message that indicates what happened.

For utilities/aircrack-ng/src/aircrack-ng.c, the following API strsep can indeed handle NULL pointers, but the program would silently go wrong. So I added an explicit error message.