Bugfix for loading multiboot kernels

This patch fixes a bug in rom_copy introduced by
commit d60fa42.

rom_copy failed to load roms with a "datasize" of 0.
As a result, multiboot kernels were not loaded correctly
when they contain a segment with a "file size" of 0.

https://bugs.launchpad.net/qemu/+bug/1208944

Signed-off-by: Martijn van den Broek <martijn.vdbrk@gmail.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Message-id: CAG1x_oET1u3TMPu3r_zzd3ZXsTWQLiaM0zAc+RkHFCwvJjGOvg@mail.gmail.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/core/loader.c

@@ -814,9 +814,6 @@ int rom_copy(uint8_t *dest, hwaddr addr, size_t size)
         if (rom->addr > end) {
             break;
         }
-        if (!rom->data) {
-            continue;
-        }
 
         d = dest + (rom->addr - addr);
         s = rom->data;
@@ -826,7 +823,9 @@ int rom_copy(uint8_t *dest, hwaddr addr, size_t size)
             l = dest - d;
         }
 
-        memcpy(d, s, l);
+        if (l > 0) {
+            memcpy(d, s, l);
+        }
 
         if (rom->romsize > rom->datasize) {
             /* If datasize is less than romsize, it means that we didn't