it doesn't matter if its signed or unsigned. if its a 32bit offset into a 64bit address space where the two pointers are greater than 2gb/4gb apart, you will have problems.
the only semi-trivial fix is to use a signed integer and to allocate *all* qc-accessible strings on the quake hunk (player names and temp strings will need to be moved). naturally this will still break many qccx hacks.
address space layout randomisation likes using the WHOLE 47-bit address space available to it - nice big gaps make it harder to guess where the actual code is, avoiding potential exploits.
I don't know about linux, but msvc has a 'smaller type check' setting that will generate some extra code that triggers a fault whenever a variable is stored into a variable that is too small for the value. this should generally be sufficient to catch pointer offset overflows, but does depend upon the result not explicitly being cast to an int. if all goes well, this gives you a fault when the subtraction happens instead of merely where the dereference happens.
linux seems to only have compile-time warnings, and will thus probably contain lots of false positives.
either way, you're actually going to have to use a debugger.