Bug 856796 - Attempt detection of YARR bug; r=till

js/src/yarr/YarrInterpreter.cpp

@@ -1009,6 +1009,12 @@ public:
                     }
                 }
             } else {
+                // Avoid a topcrash before it occurs.
+                if (!backTrack->lastContext) {
+                    ASSERT(!"Tripped Bug 856796!");
+                    return JSRegExpErrorInternal;
+                }
+
                 resetMatches(term, context);
                 popParenthesesDisjunctionContext(backTrack);
                 freeParenthesesDisjunctionContext(context);
@@ -1055,6 +1061,12 @@ public:
                     return JSRegExpMatch;
                 }
 
+                // Avoid a topcrash before it occurs.
+                if (!backTrack->lastContext) {
+                    ASSERT(!"Tripped Bug 856796!");
+                    return JSRegExpErrorInternal;
+                }
+
                 // pop a match off the stack
                 resetMatches(term, context);
                 popParenthesesDisjunctionContext(backTrack);