mboxd_flash_virtual.cpp: Fix variable name for checking mmap rc

The return code for mmap is stored in variable mapped_mem, but
then variable mem was being checked for the return code value.
Change that to mapped_mem.

Change-Id: Ie198c78f1d404a34767d7781a2fdcc48e1724a44
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/mboxd_flash_virtual.cpp b/mboxd_flash_virtual.cpp
index 26fe07a..442593b 100644
--- a/mboxd_flash_virtual.cpp
+++ b/mboxd_flash_virtual.cpp
@@ -164,7 +164,7 @@
             auto mapped_mem = mmap(NULL, partitionInfo->data.actual,
                                    PROT_READ, MAP_PRIVATE, roRequest.fd(), 0);
 
-            if (mem == MAP_FAILED)
+            if (mapped_mem == MAP_FAILED)
             {
                 MSG_ERR("Failed to map partition=%s:Error=%s\n",
                         partitionInfo->data.name, strerror(errno));