spapr_nvram: Check return value from blk_getlength()

The blk_getlength() function can return an error value if the
image size cannot be determined. Check for this rather than
ploughing on and trying to g_malloc0() a negative number.
(Spotted by Coverity, CID 1288484.)
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>
Signed-off-by: NDavid Gibson <david@gibson.dropbear.id.au>

hw/nvram/spapr_nvram.c

@@ -144,7 +144,15 @@ static void spapr_nvram_realize(VIOsPAPRDevice *dev, Error **errp)
     int ret;
 
     if (nvram->blk) {
-        nvram->size = blk_getlength(nvram->blk);
+        int64_t len = blk_getlength(nvram->blk);
+
+        if (len < 0) {
+            error_setg_errno(errp, -len,
+                             "could not get length of backing image");
+            return;
+        }
+
+        nvram->size = len;
 
         ret = blk_set_perm(nvram->blk,
                            BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE,