virCommandAddArg: Don't abort on invalid input

Commit 912c6b22fc added abort() when the
'val' parameter is NULL along with setting the error variable for the
command. We don't want to abort in this case, just set the error.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

src/util/vircommand.c

@@ -1503,7 +1503,7 @@ virCommandAddArg(virCommandPtr cmd, const char *val)
 
     if (val == NULL) {
         cmd->has_error = EINVAL;
-        abort();
+        return;
     }
 
     arg = g_strdup(val);