killall: Don't parse SIGINVAL as signal 0

Author: https://github.com/tcl3
Commit: d97614a31e
Pull-request: https://github.com/SerenityOS/serenity/pull/19173

Userland/Utilities/killall.cpp

@@ -50,7 +50,7 @@ ErrorOr<int> serenity_main(Main::Arguments arguments)
 
         if (is_ascii_alpha(arguments.argv[1][1])) {
             int value = getsignalbyname(&arguments.argv[1][1]);
-            if (value >= 0 && value < NSIG)
+            if (value > 0 && value < NSIG)
                 number = value;
         }