Fix NULL pointer dereference in the module linker on a duplicate binding

[PGZXB]

A module whose top-level binding is declared with var/function and then
re-declared by a namespace import (import * as name) is accepted, although
ECMA-262 makes duplicate top-level module binding names an early SyntaxError.
The scanner already records SCANNER_TYPE_ERR_REDECLARED for the imported name
and the named-import clause rejects it, but the namespace-import branch of
parser_parse_import_statement() never performed that check. The name is then
referenced from the module scope while absent from module_p->imports_p, and
during linking ecma_module_resolve_export() falls through to
ecma_module_resolve_import(). That function walks imports_p with an
unbounded while (true) loop whose only terminating guard is a JERRY_ASSERT
(compiled out in release builds), so it dereferences
import_node_p->module_names_p after import_node_p has reached NULL:

// m0.mjs
var z = {};
import * as z from "./m1.mjs";
export default 1;

ERROR: AddressSanitizer: SEGV on unknown address 0x000000000008
    #0 ecma_module_resolve_import jerry-core/ecma/base/ecma-module.c
    #1 ecma_module_resolve_export jerry-core/ecma/base/ecma-module.c:446
    #2 ecma_module_namespace_object_add_export_if_needed ... ecma-module.c:641

The namespace-import path now performs the same redeclaration check that
parser_module_parse_import_clause() already uses, so the duplicate binding is
rejected as a SyntaxError while the module is parsed. As defence in depth,
ecma_module_resolve_import() now stops when it runs out of import nodes and
returns false (resolution failure) instead of relying on an assertion; the
caller already handles an unsuccessful resolution.

Verified with the shipped CLI in module mode (jerry -m):

• before the change: SUMMARY: AddressSanitizer: SEGV ... in ecma_module_resolve_import
• after the change: Unhandled exception: SyntaxError, no sanitizer report
• a valid namespace import (import * as z from "./m1.mjs") still links and
  evaluates cleanly, so existing module programs are unaffected

Fixes #5292

JerryScript-DCO-1.0-Signed-off-by: PGZXB pgzxb@qq.com