linux-user/gen-vdso: Handle fseek() failure

linux-user/gen-vdso: Handle fseek() failure

Coverity points out that we don't check for fseek() failure in gen-vdso.c,
and so we might pass -1 to malloc(). Add the error checking.

(This is a standalone executable that doesn't link against glib, so
we can't do the easy thing and use g_file_get_contents().)

Coverity: CID 1523742
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-ID: <20250710170707.1299926-2-peter.maydell@linaro.org>