Prevent underflow in heap size calculation
Prevent underflow in heap size calculation

If the free memory on a device is small enough then the calculation to
determine heap size could underflow to a large value. If this happens
then malloc will never return failure and instead will cause a crash on
allocation. This patch prevents the underflow so malloc works as
expected even with low amounts of free memory.

j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os,j-greffe/mbed-os