Bug 1827359 - Add release assert to CompilationAtomCache::getExistingStringAt(). r=jandem, a=RyanVM

Differential Revision: https://phabricator.services.mozilla.com/D176031

js/src/frontend/Stencil.cpp

@@ -7,6 +7,7 @@
 #include "frontend/Stencil.h"
 
 #include "mozilla/AlreadyAddRefed.h"        // already_AddRefed
+#include "mozilla/Assertions.h"             // MOZ_RELEASE_ASSERT
 #include "mozilla/Maybe.h"                  // mozilla::Maybe
 #include "mozilla/OperatorNewExtensions.h"  // mozilla::KnownNotNull
 #include "mozilla/PodOperations.h"          // mozilla::PodCopy
@@ -4063,6 +4064,7 @@ void ExtensibleCompilationStencil::dumpAtom(TaggedParserAtomIndex index) {
 
 JSString* CompilationAtomCache::getExistingStringAt(
     ParserAtomIndex index) const {
+  MOZ_RELEASE_ASSERT(atoms_.length() >= index);
   return atoms_[index];
 }