misc: Use g_assert_not_reached for code which is expected to be unreachable

The macro g_assert_not_reached is a better self documenting replacement
for assert(0) or assert(false).

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
diff --git a/blockdev.c b/blockdev.c
index 4534864..7879e85 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1029,7 +1029,7 @@
 
 static void abort_commit(BlkTransactionState *common)
 {
-    assert(false); /* this action never succeeds */
+    g_assert_not_reached(); /* this action never succeeds */
 }
 
 static const BdrvActionOps actions[] = {