kernel/user_debugger: Zero-initialize addressOffset.

On some optimization levels we get warnings/errors about it being
potentially uninitialized (but not at -O2, hence this wasn't a
problem in nightly builds.)

src/system/kernel/debug/user_debugger.cpp

@@ -1849,7 +1849,7 @@ debug_nub_thread(void *)
 
 				// find the area
 				area_id sourceArea;
-				addr_t addressOffset;
+				addr_t addressOffset = 0;
 				if (result == B_OK) {
 					sourceArea = _user_area_for((void*)address);
 					if (sourceArea < 0) {