memory: return MemoryRegion from qemu_ram_addr_from_host

It will be needed in the next patch.

Reviewed-by: Jan Kiszka <jan.kiszka@siemens.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
diff --git a/cputlb.c b/cputlb.c
index be24080..828007c 100644
--- a/cputlb.c
+++ b/cputlb.c
@@ -162,7 +162,7 @@
 {
     ram_addr_t ram_addr;
 
-    if (qemu_ram_addr_from_host(ptr, &ram_addr)) {
+    if (qemu_ram_addr_from_host(ptr, &ram_addr) == NULL) {
         fprintf(stderr, "Bad ram pointer %p\n", ptr);
         abort();
     }