Fix #6720 and #6721 (Crashes on garbage code)

Local fixes to avoid access to NULL-token

lib/checkbufferoverrun.cpp

@@ -1721,6 +1721,8 @@ void CheckBufferOverrun::arrayIndexThenCheck()
                 if (tok->type() == Token::eComparisonOp)
                     tok = tok->tokAt(2);
 
+				if (!tok)
+                    break;
                 // skip close parentheses
                 if (tok->str() == ")")
                     tok = tok->next();

lib/checkstring.cpp

@@ -98,6 +98,8 @@ void CheckString::checkAlwaysTrueOrFalseStringCompare()
             alwaysTrueFalseStringCompareError(tok, str1, str2);
             tok = tok->tokAt(5);
         }
+		if (!tok)
+			break;
     }
 }
 

test/testgarbage.cpp

@@ -90,6 +90,8 @@ private:
         TEST_CASE(garbageCode49); // #6715
         TEST_CASE(garbageCode50); // #6718
         TEST_CASE(garbageCode51); // #6719
+		TEST_CASE(garbageCode52); // #6720
+		TEST_CASE(garbageCode53); // #6721
 
         TEST_CASE(garbageValueFlow);
         TEST_CASE(garbageSymbolDatabase);
@@ -515,6 +517,14 @@ private:
         checkCode(" (const \"C\" ...); struct base { int f2; base (int arg1, int arg2); }; global_base(0x55, 0xff); { ((global_base.f1 0x55) (global_base.f2 0xff)) { } } base::base(int arg1, int arg2) { f2 = }");
     }
 
+	void garbageCode52() { // #6720
+        checkCode("a \"b\" not_eq \"c\"");
+    }
+
+	void garbageCode53() { // #6721
+        checkCode("{ { } }; void foo (struct int i) { x->b[i] = = }");
+    }
+
     void garbageValueFlow() {
         // #6089
         const char* code = "{} int foo(struct, x1, struct x2, x3, int, x5, x6, x7)\n"