clk: zynq: Prevent null pointer dereference caused by kmalloc failure

In the Linux kernel, the following vulnerability has been resolved:

clk: zynq: Prevent null pointer dereference caused by kmalloc failure

The kmalloc() in zynq_clk_setup() will return null if the

physical memory has run out. As a result, if we use snprintf()

to write data to the null address, the null pointer dereference

bug will happen.

This patch uses a stack variable to replace the kmalloc().