tester.c: fix resource directory path

tester/mediastreamer2_tester.c

@@ -89,12 +89,12 @@ int main (int argc, char *argv[]) {
 
 	mediastreamer2_tester_init(NULL);
 
+	// this allows to launch liblinphone_tester from outside of tester directory
 	if (strstr(argv[0], ".libs")) {
-		char res_dir[128] = {0};
-		// this allows to launch liblinphone_tester from outside of tester directory
-		strncpy(res_dir, argv[0], strstr(argv[0], ".libs")-argv[0]);
-		bc_tester_set_resource_dir_prefix(res_dir);
-		bc_tester_set_writable_dir_prefix(res_dir);
+		char* prefix = ms_strdup_printf("%s%s", argv[0][0]=='/'?"":"./", strstr(argv[0], ".libs"));
+		bc_tester_set_resource_dir_prefix(prefix);
+		bc_tester_set_writable_dir_prefix(prefix);
+		ms_free(prefix);
 	}
 
 	for(i = 1; i < argc; ++i) {