xen/arm: static-shmem: fix "gbase/pbase used uninitialized" build failure

Building Xen with CONFIG_STATIC_SHM=y results in a build failure:

arch/arm/static-shmem.c: In function 'process_shm':
arch/arm/static-shmem.c:327:41: error: 'gbase' may be used uninitialized [-Werror=maybe-uninitialized]
  327 |         if ( is_domain_direct_mapped(d) && (pbase != gbase) )
arch/arm/static-shmem.c:305:17: note: 'gbase' was declared here
  305 |         paddr_t gbase, pbase, psize;

This is because the commit cb1ddafdc5 adds a check referencing
gbase/pbase variables which were not yet assigned a value. Fix it.

Fixes: cb1ddafdc5 ("xen/arm/static-shmem: Static-shmem should be direct-mapped for direct-mapped domains")
Signed-off-by: Michal Orzel <michal.orzel@amd.com>
Release-Acked-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>

xen/arch/arm/static-shmem.c

@@ -324,12 +324,6 @@ int __init process_shm(struct domain *d, struct kernel_info *kinfo,
             printk("%pd: static shared memory bank not found: '%s'", d, shm_id);
             return -ENOENT;
         }
-        if ( is_domain_direct_mapped(d) && (pbase != gbase) )
-        {
-            printk("%pd: physical address 0x%"PRIpaddr" and guest address 0x%"PRIpaddr" are not direct-mapped.\n",
-                   d, pbase, gbase);
-            return -EINVAL;
-        }
 
         pbase = boot_shm_bank->start;
         psize = boot_shm_bank->size;
@@ -353,6 +347,13 @@ int __init process_shm(struct domain *d, struct kernel_info *kinfo,
             /* guest phys address is after host phys address */
             gbase = dt_read_paddr(cells + addr_cells, addr_cells);
 
+            if ( is_domain_direct_mapped(d) && (pbase != gbase) )
+            {
+                printk("%pd: physical address 0x%"PRIpaddr" and guest address 0x%"PRIpaddr" are not direct-mapped.\n",
+                       d, pbase, gbase);
+                return -EINVAL;
+            }
+
             for ( i = 0; i < PFN_DOWN(psize); i++ )
                 if ( !mfn_valid(mfn_add(maddr_to_mfn(pbase), i)) )
                 {