cmd: ufetch: Initialise size before first use

cmd: ufetch: Initialise size before first use

The local variable size is not assigned to before it is used
for the first time. Correct this.

This issue was found by Smatch.

Fixes: 86d462c05d57 (cmd: add a fetch utility)
Signed-off-by: Andrew Goodbody <andrew.goodbody@linaro.org>
Reviewed-by: Casey Connolly <casey.connolly@linaro.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Link: https://lore.kernel.org/r/20250626-ufetch_fix-v1-1-025afdb85dc2@linaro.org
Signed-off-by: Casey Connolly <casey.connolly@linaro.org>