MdeModulePkg: RegularExpressionDxe: support free(NULL)
The ISO C standard says about free(),
If ptr is a null pointer, no action occurs.
This is not true of the FreePool() interface of the MemoryAllocationLib
class:
Buffer must have been allocated on a previous call to the pool
allocation services of the Memory Allocation Library. [...] If Buffer
was not allocated with a pool allocation function in the Memory
Allocation Library, then ASSERT().
Therefore we must not forward the argument of free() to FreePool() without
checking.