Fix wrong size calculation for "Dx ?" larger than DB

The size calculation done in len_extops() (called by insn_size()) for
EOT_DB_RESERVE (i.e. uninitialized storage "?" token) does not take
into account the element size (e->elem), thus calculating a wrong
size for any Dx larger than DB (DW, DQ, etc).

The bug is silent, but it makes NASM error out if a "Dx ?" (larger
than DB) is followed by any label because the label offset gets
mismatched in the final code generation stage:

    $ cat test.asm
    [section .bss]
    DW ?
    x:

    $ nasm test.asm
    test.asm:3: error: label `x' changed during code generation [-w+error=label-redef-late]

See also: https://stackoverflow.com/q/70012188/3889449

Signed-off-by: Marco Bonelli <marco@mebeim.net>
Signed-off-by: Cyrill Gorcunov <gorcunov@gmail.com>

asm/assemble.c

@@ -1111,7 +1111,7 @@ static int64_t len_extops(const extop *e)
             break;
 
         case EOT_DB_RESERVE:
-            isize += e->dup;
+            isize += e->dup * e->elem;
             break;
         }