libbb/u_signal_names.c: don't check errno after bb_strtou

libbb/u_signal_names.c: don't check errno after bb_strtou

Since we're comparing the return value to a smallish integer anyway, we
might as well use that bb_strtou() returns UINT_MAX for malformed
input. Referencing errno is kinda bloaty on glibc.

While NSIG is not in POSIX, we do already rely on it being defined,
compile-time const and smallish, since arrays in struct globals_misc are
defined in terms of it.

function                                             old     new   delta
get_signum                                           312     286     -26

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>