Prevent offset + size overflow.

it is possible to overflow uint64_t by summing variables offset and
size up in elf_get_section_info. Thee values are extracted from module
file and are possibly maliciously tampered with.

If offset is in valid range and size very large, the result will
overflow and the size check passes. Later on, this will most likely
lead to a segmentation fault due to accessing uninitialized memory.

Attached please find a proof of concept module, which will trigger
a segmentation fault on modinfo. Tested on amd64:

tobias:~$ modinfo poc.ko
filename:       /home/tobias/poc.ko
Segmentation fault

There are more errors of this type in the ELF handling code that will be
fixed in other patches.

libkmod/libkmod-elf.c

@@ -251,7 +251,7 @@ static inline int elf_get_section_info(const struct kmod_elf *elf, uint16_t idx,
 #undef READV
 
 	min_size = *offset + *size;
-	if (min_size > elf->size) {
+	if (ULLONG_MAX - *offset < *size || min_size > elf->size) {
 		ELFDBG(elf, "out-of-bounds: %"PRIu64" >= %"PRIu64" (ELF size)\n",
 		       min_size, elf->size);
 		return -EINVAL;