Allow GetPhysicalRef to hold a past-the-end offset (#6141)

Games will sometimes use these when representing open right bounds
and so disallowing it caused regressions, with a notable example
being when MemoryFill is called to the end of vram, causing an
"invalid end address" error.
This had been noted on a comment in GetPhysicalRef prior to the
regression.

src/core/memory.cpp

@@ -496,7 +496,7 @@ MemoryRef MemorySystem::GetPhysicalRef(PAddr address) const {
     default:
         UNREACHABLE();
     }
-    if (offset_into_region >= target_mem->GetSize()) {
+    if (offset_into_region > target_mem->GetSize()) {
         return {nullptr};
     }