hw/alpha/dp264.c: Fix memory leak spotted by valgrind

valgrind complains about:
==7055== 58 bytes in 1 blocks are definitely lost in loss record 1,471 of 2,192
==7055==    at 0x4C2845D: malloc (in /usr/lib64/valgrind/vgpreload_memcheck-amd64-linux.so)
==7055==    by 0x24410F: malloc_and_trace (vl.c:2556)
==7055==    by 0x64C770E: g_malloc (in /usr/lib64/libglib-2.0.so.0.3600.3)
==7055==    by 0x64DEFD7: g_strndup (in /usr/lib64/libglib-2.0.so.0.3600.3)
==7055==    by 0x650181A: g_vasprintf (in /usr/lib64/libglib-2.0.so.0.3600.3)
==7055==    by 0x64DF0CC: g_strdup_vprintf (in /usr/lib64/libglib-2.0.so.0.3600.3)
==7055==    by 0x64DF188: g_strdup_printf (in /usr/lib64/libglib-2.0.so.0.3600.3)
==7055==    by 0x242F81: qemu_find_file (vl.c:2121)
==7055==    by 0x217A32: clipper_init (dp264.c:105)
==7055==    by 0x2484DA: main (vl.c:4249)

Signed-off-by: Shannon Zhao <zhaoshenglong@huawei.com>
Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
