BUG/MINOR: sym: don't crash on images linked at a fixed base

BUG/MINOR: sym: don't crash on images linked at a fixed base

collect_dynsym_mem() biases the dynamic entries below the load
address, but those of an image linked at a fixed base sit far above
it and are used as is: this is the case of the vDSO image that old
kernels (e.g. 3.10) build prelinked, and haproxy then crashes at
start-up, the tables being read through unmapped pointers.
Fix that by trying to figure out if the pointer was already relocated
or not.