Fix for segfault after removing PrefetchRange from map

The start iterator becomes invalid after it is removed from
std::map prefetch_map_. This was causing a segfault when the iterator is
incremented afterwards.

Signed-off-by: David Yat Sin <david.yatsin@amd.com>
Change-Id: I4b0b763d2cb4ee99c0b8571c2c526b834e74077a

runtime/hsa-runtime/core/runtime/runtime.cpp

@@ -2030,7 +2030,8 @@ hsa_status_t Runtime::SvmPrefetch(void* ptr, size_t size, hsa_agent_t agent,
             start->second.prev->second.next = start->second.next;
             if (!isEndNode(start)) start->second.next->second.prev = start->second.prev;
           }
-          prefetch_map_.erase(start);
+          start = prefetch_map_.erase(start);
+          continue;
         }
       }
       start++;