efiemu: fix unintended sign extension

Found by: Coverity scan.
CID: 73883, 73637

grub-core/efiemu/loadcore.c

@@ -343,7 +343,7 @@ SUFFIX (grub_efiemu_loadcore_init) (void *core, const char *filename,
     return grub_error (GRUB_ERR_BAD_MODULE, N_("this ELF file is not of the right type"));
 
   /* Make sure that every section is within the core.  */
-  if ((grub_size_t) core_size < e->e_shoff + e->e_shentsize * e->e_shnum)
+  if ((grub_size_t) core_size < e->e_shoff + (grub_uint32_t) e->e_shentsize * e->e_shnum)
     return grub_error (GRUB_ERR_BAD_OS, N_("premature end of file %s"),
 		       filename);