exec: fix early return from ram_block_add

After reporting an error, ram_block_add was going on with the registration
of the RAMBlock.  The visible effect is that it unlocked the ramlist
mutex twice.

Fixes: 528f46af6e
Reviewed-by: Fam Zheng <famz@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

exec.c

@@ -1589,6 +1589,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
             if (err) {
                 error_propagate(errp, err);
                 qemu_mutex_unlock_ramlist();
+                return;
             }
         } else {
             new_block->host = phys_mem_alloc(new_block->max_length,
@@ -1598,6 +1599,7 @@ static void ram_block_add(RAMBlock *new_block, Error **errp)
                                  "cannot set up guest memory '%s'",
                                  memory_region_name(new_block->mr));
                 qemu_mutex_unlock_ramlist();
+                return;
             }
             memory_try_enable_merging(new_block->host, new_block->max_length);
         }