Fix use-after-free crash in iTermProcessMonitor

When child monitors are removed from the tree, their dispatch sources
must be invalidated before the monitor is deallocated. Without this,
the dispatch source is still active when the monitor is freed, causing
a crash in _dispatch_queue_xref_dispose.

Author: chall37

sources/iTermProcessMonitor.m

@@ -116,6 +116,7 @@ - (BOOL)setProcessInfo:(iTermProcessInfo *)processInfo depth:(NSInteger)depth {
         [self addChild:child];
     }];
     [childrenToRemove enumerateObjectsUsingBlock:^(iTermProcessMonitor * _Nonnull child, NSUInteger idx, BOOL * _Nonnull stop) {
+        [child invalidate];
         [self removeChild:child];
     }];
     if (childrenToAdd.count || childrenToRemove.count) {