util: storage: Fix build after 25924dec

The commit referenced above changed function arguments of
virStorageFileGetMetadataFromBuf() but didn't tweak the
ATTRIBUTE_NONNULL tied to them. This was caught by coverity as it
actually obeys them. We disabled them for GCC and thus it didn't show
up.

Additionally in commit 3ea661de I passed
NULL to the backingFormat argument which was also marked as nonnull. Use
a dummy int's address when the argument isn't supplied so that the code
doesn't need to change much.

src/util/virstoragefile.c

@@ -955,6 +955,10 @@ virStorageFileGetMetadataFromBuf(const char *path,
                                  int *backingFormat)
 {
     virStorageSourcePtr ret = NULL;
+    int dummy;
+
+    if (!backingFormat)
+        backingFormat = &dummy;
 
     if (!(ret = virStorageFileMetadataNew(path, format)))
         return NULL;

src/util/virstoragefile.h

@@ -301,8 +301,7 @@ virStorageSourcePtr virStorageFileGetMetadataFromBuf(const char *path,
                                                      size_t len,
                                                      int format,
                                                      int *backingFormat)
-    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2) ATTRIBUTE_NONNULL(4)
-    ATTRIBUTE_NONNULL(5);
+    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 int virStorageFileChainGetBroken(virStorageSourcePtr chain,
                                  char **broken_file);