storagefile: Fix backing format \0 check

From qemu.git docs/interop/qcow2.txt

  == String header extensions ==

  Some header extensions (such as the backing file format name and
  the external data file name) are just a single string. In this case,
  the header extension length is the string length and the string is
  not '\0' terminated. (The header extension padding can make it look
  like a string is '\0' terminated, but neither is padding always
  necessary nor is there a guarantee that zero bytes are used
  for padding.)

So we shouldn't be checking for a \0 byte at the end of the backing
format section. I think in practice there always is a \0 but we
shouldn't depend on that.
Reviewed-by: NMichal Privoznik <mprivozn@redhat.com>
Signed-off-by: NCole Robinson <crobinso@redhat.com>

src/util/virstoragefile.c

@@ -503,15 +503,21 @@ qcow2GetExtensions(const char *buf,
             break;
 
         switch (magic) {
-        case QCOW2_HDR_EXTENSION_END:
-            goto done;
+        case QCOW2_HDR_EXTENSION_BACKING_FORMAT: {
+            VIR_AUTOFREE(char *) tmp = NULL;
+            if (VIR_ALLOC_N(tmp, len + 1) < 0)
+                return -1;
+            memcpy(tmp, buf + offset, len);
+            tmp[len] = '\0';
 
-        case QCOW2_HDR_EXTENSION_BACKING_FORMAT:
-            if (buf[offset+len] != '\0')
-                break;
-            *backingFormat = virStorageFileFormatTypeFromString(buf+offset);
+            *backingFormat = virStorageFileFormatTypeFromString(tmp);
             if (*backingFormat <= VIR_STORAGE_FILE_NONE)
                 return -1;
+            break;
+        }
+
+        case QCOW2_HDR_EXTENSION_END:
+            goto done;
         }
 
         offset += len;