policy: check for null instead of falsy in loop

This prepares the code for the no-cond-assign ESLint rule.

PR-URL: #41614
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: Trott

lib/internal/policy/sri.js

@@ -38,7 +38,7 @@ const parse = (str) => {
   let prevIndex = 0;
   let match;
   const entries = [];
-  while (match = RegExpPrototypeExec(kSRIPattern, str)) {
+  while ((match = RegExpPrototypeExec(kSRIPattern, str)) !== null) {
     if (match.index !== prevIndex) {
       throw new ERR_SRI_PARSE(str, str[prevIndex], prevIndex);
     }