Correct typo in malloc debug environment variable (#2391)

I don't think this is currently used for anything. While the malloc-test machinery is working atm, some errors won't be caught due to the typo. https://man7.org/linux/man-pages/man3/mallopt.3.html says it should be MALLOC_CHECK_.

Author: torben-hansen

ssl/test/runner/runner.go

@@ -1658,7 +1658,7 @@ func runTest(dispatcher *shimDispatcher, statusChan chan statusMsg, test *testCa
 		if *mallocTestDebug {
 			env = append(env, "MALLOC_BREAK_ON_FAIL=1")
 		}
-		env = append(env, "_MALLOC_CHECK=1")
+		env = append(env, "MALLOC_CHECK_=1")
 	}
 
 	shim, err := newShimProcess(dispatcher, shimPath, flags, env)

util/all_tests.go

@@ -207,7 +207,7 @@ func runTestOnce(test test, mallocNumToFail int64) (passed bool, err error) {
 		if *mallocTestDebug {
 			cmd.Env = append(cmd.Env, "MALLOC_ABORT_ON_FAIL=1")
 		}
-		cmd.Env = append(cmd.Env, "_MALLOC_CHECK=1")
+		cmd.Env = append(cmd.Env, "MALLOC_CHECK_=1")
 	}
 
 	if err := cmd.Start(); err != nil {