vdi: Fix warning from clang

ccc-analyzer reports these warnings:

block/vdi.c:704:13: warning: Dereference of null pointer
            bmap[i] = VDI_UNALLOCATED;
            ^
block/vdi.c:702:13: warning: Dereference of null pointer
            bmap[i] = i;
            ^

Moving some code into the if block fixes this.
It also avoids calling function write with 0 bytes of data.
Signed-off-by: NStefan Weil <sw@weilnetz.de>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

block/vdi.c

@@ -628,7 +628,6 @@ static int vdi_create(const char *filename, QEMUOptionParameter *options)
     VdiHeader header;
     size_t i;
     size_t bmap_size;
-    uint32_t *bmap;
 
     logout("\n");
 
@@ -693,10 +692,8 @@ static int vdi_create(const char *filename, QEMUOptionParameter *options)
         result = -errno;
     }
 
-    bmap = NULL;
     if (bmap_size > 0) {
-        bmap = (uint32_t *)g_malloc0(bmap_size);
-    }
+        uint32_t *bmap = g_malloc0(bmap_size);
         for (i = 0; i < blocks; i++) {
             if (image_type == VDI_TYPE_STATIC) {
                 bmap[i] = i;
@@ -708,6 +705,8 @@ static int vdi_create(const char *filename, QEMUOptionParameter *options)
             result = -errno;
         }
         g_free(bmap);
+    }
+
     if (image_type == VDI_TYPE_STATIC) {
         if (ftruncate(fd, sizeof(header) + bmap_size + blocks * block_size)) {
             result = -errno;