livetree: Fix a comparison of integers with different signedness

[ukleinek]

On some architectures we have:

livetree.c: In function 'fixup_phandles':
livetree.c:1237:54: error: comparison of integer expressions of different signedness: 'long int' and 'unsigned int' [-Werror=sign-compare]
 1237 |                         if (offset < 0 || offset + 4 > p->val.len) {
      |                                                      ^
cc1: all warnings being treated as errors
make: *** [Makefile:307: livetree.o] Error 1

Fixes: a26ef64 ("Restore phandle references from fixups node")
Closes: #186

[ukleinek]

BTW, gcc 15.2.0 on x86_64 is fine with the code, gcc 15.2.0 on armhf triggers this warning. I guess the relevant difference is that on x86_64 an unsigned long (64bit) is big enough to hold all values of an unsigned int (32bit), but on armhf (both 32 bit) it isn't.