util: bitmap: Tolerate NULL bitmaps in virBitmapEqual

After virBitmapEqual is able to compare NULL bitmaps few bits of code
can be cleaned up.

(cherry picked from commit 20448c2a)
Signed-off-by: NJán Tomko <jtomko@redhat.com>

Partial backport without the cleanup.

src/util/virbitmap.c

@@ -504,6 +504,12 @@ bool virBitmapEqual(virBitmapPtr b1, virBitmapPtr b2)
     virBitmapPtr tmp;
     size_t i;
 
+    if (!b1 && !b2)
+        return true;
+
+    if (!b1 || !b2)
+        return false;
+
     if (b1->max_bit > b2->max_bit) {
         tmp = b1;
         b1 = b2;

src/util/virbitmap.h

@@ -84,8 +84,7 @@ virBitmapPtr virBitmapNewData(void *data, int len) ATTRIBUTE_NONNULL(1);
 int virBitmapToData(virBitmapPtr bitmap, unsigned char **data, int *dataLen)
     ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
 
-bool virBitmapEqual(virBitmapPtr b1, virBitmapPtr b2)
-    ATTRIBUTE_NONNULL(1) ATTRIBUTE_NONNULL(2);
+bool virBitmapEqual(virBitmapPtr b1, virBitmapPtr b2);
 
 size_t virBitmapSize(virBitmapPtr bitmap)
     ATTRIBUTE_NONNULL(1);