Fix adb get-serialno parsing

The function assumed that the raw output of "adb get-serialno" was a
NUL-terminated string, but it is not the case.

It this output did not end with a space or a new line character, then
sc_str_truncate() would write '\0' over the last character. Even worse,
if the output was empty, then sc_str_truncate() would write
out-of-bounds.

Avoid the error-prone sc_str_truncate() util function.