libICE : Dereferencing a possible NULL pointer in error.c

In function _IceErrorSetupFailed at line no 188, IceAllocScratch is used to
allocate memory for variable pStart and pBuf.

IceAllocScratch is using malloc for memory allocation, malloc can return
NULL on failure. So before dereferencing pStart and pBuf, NULL check must
apply.

v2 (by alanc): correct inverted tests for NULL from original patch

Closes: #5
Signed-off-by: mahendra <mahendra.n@samsung.com>
Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Part-of: <!22>