Added checks for index in ThreadLocalStorage APIs (FreeRTOS#552)

Added checks for ( xIndex >= 0 ) in ThreadLocalStorage APIs

Author: AniruddhaKanhere

tasks.c

@@ -4907,7 +4907,8 @@ static portTASK_FUNCTION( prvIdleTask, pvParameters )
     {
         TCB_t * pxTCB;
 
-        if( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS )
+        if( ( xIndex >= 0 ) &&
+            ( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS ) )
         {
             pxTCB = prvGetTCBFromHandle( xTaskToSet );
             configASSERT( pxTCB != NULL );
@@ -4926,7 +4927,8 @@ static portTASK_FUNCTION( prvIdleTask, pvParameters )
         void * pvReturn = NULL;
         TCB_t * pxTCB;
 
-        if( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS )
+        if( ( xIndex >= 0 ) &&
+            ( xIndex < configNUM_THREAD_LOCAL_STORAGE_POINTERS ) )
         {
             pxTCB = prvGetTCBFromHandle( xTaskToQuery );
             pvReturn = pxTCB->pvThreadLocalStoragePointers[ xIndex ];