Fix -Wmaybe-uninitialized warning in opcodes/i386-dis.c

Fix -Wmaybe-uninitialized warning in opcodes/i386-dis.c

A recent change in opcodes/i386-dis.c caused a build failure on my
x86-64 Fedora 36 system, which uses:

$ gcc --version
gcc (GCC) 12.2.1 20221121 (Red Hat 12.2.1-4)
[...]

The error is:

../../binutils-gdb/opcodes/i386-dis.c: In function ‘OP_J’:
../../binutils-gdb/opcodes/i386-dis.c:12705:22: error: ‘val’ may be used uninitialized [-Werror=maybe-uninitialized]
12705 |           disp = val & 0x8000 ? val - 0x10000 : val;
      |                  ~~~~^~~~~~~~

This patch fixes the warning.

opcodes/ChangeLog
2023-04-21  Tom Tromey  <tromey@adacore.com>

	* i386-dis.c (OP_J): Check result of get16.