Debugger: Speed up disassembly retrieval

This skips the case of "disassembling a function given an address"
with indeterministic time behaviour and and immediately falls back
to the fixed range version instead.

Change-Id: I90ec56198a7d767bcf0a9ffb030637c2ee5c617a
Reviewed-by: Kai Koehne <kai.koehne@digia.com>