free: Parse -s option correctly.

If the -s option was the first option on the command line, free
would report seconds argument failed. This only appeared on the
Debian free, not the one in git.

Closer examination revealed that if a valid float string is
given to strtof() it doesn't set errno to 0, but just leaves it
alone. As we are explicitly testing errno for overflows, this
means the previous errno change is picked up here.

The simple answer is to set errno to 0 before calling strtof().

References:
 https://bugs.debian/org/733758
 https://enc.com.au/2015/08/08/be-careful-with-errno/

NEWS

@@ -12,6 +12,7 @@ procps-ng-NEXT
   * top once again will fully honor a saved rcfile,
     without requiring --disable-modern-top. Debian #762928, #762947
   * vmstat: Not crash if partition appears before disk Debian #736628
+  * free: -s without -c works Debian #733758
 
 procps-ng-3.3.10
 ----------------

free.c

@@ -323,6 +323,7 @@ int main(int argc, char **argv)
 			break;
 		case 's':
 			flags |= FREE_REPEAT;
+			errno = 0;
 			args.repeat_interval = (1000000 * strtof(optarg, &endptr));
 			if (errno || optarg == endptr || (endptr && *endptr))
 				xerrx(EXIT_FAILURE, _("seconds argument `%s' failed"), optarg);