ly_path_parse OPTIMIZE fail faster on invalid path

`ly_path_parse()` is called from various functions like
`lyplg_type_store_node_instanceid()` and `lyplg_type_lypath_new()` with
the `LY_PATH_BEGIN_ABSOLUTE` flag set.

This means that the path is expected to start with a '/' character.
If it doesn't, this could mean that the value is not a valid path.

This can happen while finding a union type, where it is not optimal to
call `lyxp_expr_parse()`, which is much heavier to compute.

Author: irfanHaslanded

src/path.c

@@ -335,6 +335,9 @@ ly_path_parse(const struct ly_ctx *ctx, const struct lysc_node *ctx_node, const
             (prefix == LY_PATH_PREFIX_FIRST) || (prefix == LY_PATH_PREFIX_STRICT_INHERIT));
     assert((pred == LY_PATH_PRED_KEYS) || (pred == LY_PATH_PRED_SIMPLE) || (pred == LY_PATH_PRED_LEAFREF));
 
+    /* path_len must be specified or begin cannot be absolute */
+    assert(path_len || (begin != LY_PATH_BEGIN_ABSOLUTE));
+
     if (ctx_node) {
         LOG_LOCSET(ctx_node, NULL);
     }
@@ -343,6 +346,14 @@ ly_path_parse(const struct ly_ctx *ctx, const struct lysc_node *ctx_node, const
         path_len = strlen(str_path);
     }
 
+    /* check if path begins with '/' if expected to and fail early if not */
+    if ((begin == LY_PATH_BEGIN_ABSOLUTE) && (str_path[0] != '/')) {
+        LOGVAL(ctx, LYVE_XPATH, "Unexpected XPath token \"%c\" (\"%.*s\"), expected \"Operator(Path)\".",
+                str_path[0], (int)path_len, str_path);
+        ret = LY_EVALID;
+        goto error;
+    }
+
     /* parse as a generic XPath expression, reparse is performed manually */
     LY_CHECK_GOTO(ret = lyxp_expr_parse(ctx, str_path, path_len, 0, &exp), error);
     tok_idx = 0;