Fix segfault on %{?getncpus}, regression in 4.19.x

Commit deaebd0c89 assumed there would
always be args, but in the macro existence test none are set up.

Reported as a side-issue in https://issues.redhat.com/browse/RHEL-16557

rpmio/macro.c

@@ -1219,7 +1219,7 @@ static unsigned long getmem_proc(int thread)
 static void doGetncpus(rpmMacroBuf mb, rpmMacroEntry me, ARGV_t argv, size_t *parsed)
 {
     const char *sizemacro = NULL;
-    const char *arg = argv[1] ? argv[1] : "total";
+    const char *arg = (argv && argv[1]) ? argv[1] : "total";
     char buf[32];
     unsigned int ncpus = getncpus();
     unsigned long mem = 0;