tidy: Explicitly check the return value of some system calls.

These problems were pointed out by clang-tidy's "implicit boolean
conversion" check.  Changes made by the clang-tidy program.

https://clang.llvm.org/extra/clang-tidy/checks/readability-implicit-bool-conversion.html

mythtv/libs/libmythbase/mythsystemlegacy.cpp

@@ -157,7 +157,7 @@ void MythSystemLegacy::SetCommand(const QString &command,
     }
 
     // check for execute rights
-    if (!GetSetting("UseShell") && access(command.toUtf8().constData(), X_OK))
+    if (!GetSetting("UseShell") && (access(command.toUtf8().constData(), X_OK)) != 0)
     {
         LOG(VB_GENERAL, LOG_ERR,
             QString("MythSystemLegacy(%1) command not executable, ")