Fix a missing init value warning caught by static analysis.

src/extent.c

@@ -825,6 +825,7 @@ extent_try_coalesce_impl(tsdn_t *tsdn, pac_t *pac, ehooks_t *ehooks,
     ecache_t *ecache, edata_t *edata, bool *coalesced) {
 	assert(!edata_guarded_get(edata));
 	assert(coalesced != NULL);
+	*coalesced = false;
 	/*
 	 * We avoid checking / locking inactive neighbors for large size
 	 * classes, since they are eagerly coalesced on deallocation which can