Enhance heap_4.c to prevent blocks accidentally being freed twice, or blocks that...

Enhance heap_4.c to prevent blocks accidentally being freed twice, or blocks that don't have a valid block link descriptor being freed.
Update the Cortex-A9 port to include asserts if an ISR safe FreeRTOS function is called from an interrupt that has a higher logical priority than configMAX_SYSCALL_INTERRUPT_PRIORITY (or whatever the CA9 equivalent is called), and also assert if the binary point is not set correctly.

git-svn-id: https://svn.code.sf.net/p/freertos/code/trunk@1967 1d2547de-c912-0410-9cb9-b8ca96c0e9e2