mkimg: Check for realloc failure

[cperciva]

Summary

If realloc fails, clean up and return ENOMEM; don't just copy data
into NULL.

Reported by: Claude Opus 5.5
Sponsored by: Amazon

[ngie]

usr.bin/mkimg/vmdk.c

136–138

This is the same thing with reallocf (which exists to avoid forgotten free calls).

[cperciva]

usr.bin/mkimg/vmdk.c

136–138

Yeah, but reallocf is non-portable so I wasn't 100% sure if it was safe to use in a bootstrap tool. I figured it was easier to write it out.

[imp]

usr.bin/mkimg/vmdk.c

136–138

We generally assume we can use any FreeBSD libc function, even in our bootstrap and add new freebsd-specific bootstrap functions to the bootstrap glue we create for !FreeBSD builds.