BUG/MEDIUM: log: fix possible segfault during config parsing

When log format arguments are specified within braces with no '+' nor '-'
prefix, the NULL string is compared with known keywords causing a crash.
This only happens during parsing so it does not affect runtime processing.
diff --git a/src/log.c b/src/log.c
index 7483842..e5d4ce8 100644
--- a/src/log.c
+++ b/src/log.c
@@ -184,7 +184,7 @@
 
 		if (*args == '\0' || *args == ',') {
 			*args = '\0';
-			for (i = 0; var_args_list[i].name; i++) {
+			for (i = 0; sp && var_args_list[i].name; i++) {
 				if (strcmp(sp, var_args_list[i].name) == 0) {
 					if (flags == 1) {
 						node->options |= var_args_list[i].mask;
@@ -199,7 +199,7 @@
 			if (end)
 				break;
 		}
-	args++;
+		args++;
 	}
 	return 0;
 }