Consider the following code:
X * p = kmalloc(sizeof(X), GFP_KERNEL);
p->x = 42;

vs

X * p = kmalloc(sizeof(X), GFP_KERNEL);
if(p) {
 p->x = 42;
}

if "kmalloc" were to return "NULL" this would write "42" to NULL+the offset in X of "x"...probably something like null+4. Running in kernel mode in low resource conditions this could result in a kernel panic or an otherwise unstable system.

It is generally considered best practice to test for NULL before dereferencing dynamically allocated memory. While auditing linux codes, the following examples of unsafe dynamic memory allocation using kmalloc were discovered.

C:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\arch\arm\mach-u300\dummyspichip.c:64 	bigrxbuf_virtual = kmalloc(DMA_TEST_SIZE, GFP_KERNEL);
C:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\arch\cris\arch-v32\kernel\signal.c:543 	u16* data = kmalloc(PAGE_SIZE, GFP_KERNEL);
C:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\arch\cris\arch-v32\mm\intmem.c:75 					tmp = kmalloc(sizeof *tmp, GFP_ATOMIC);
C:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\arch\x86\platform\efi\efi_64.c:92 	save_pgd = kmalloc(n_pgds * sizeof(pgd_t), GFP_KERNEL);
C:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\arch\x86\platform\uv\tlb_uv.c:2078 	vp = kmalloc(nuvhubs * sizeof(struct uvhub_desc), GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\misc\lkdtm.c : 383 		u32 *data = kmalloc(len, GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\parisc\ccio-dma.c : 1408 	char *name = kmalloc(14, GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\pci\msi.c : 546 		char *name = kmalloc(20, GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\staging\et131x\et131x.c : 2125 	rx_ring->fbr[0] = kmalloc(sizeof(struct fbr_lookup), GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\staging\et131x\et131x.c : 2126 	rx_ring->fbr[1] = kmalloc(sizeof(struct fbr_lookup), GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\staging\ft1000\ft1000-usb\ft1000_hw.c : 332 	commandbuf = kmalloc(size + 2, GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\drivers\staging\netlogic\xlr_net.c : 893 	priv->mii_bus->irq = kmalloc(sizeof(int)*PHY_MAX_ADDR, GFP_KERNEL);
c:\me\src\linux-3.14-rc1.tar\linux-3.14-rc1\linux-3.14-rc1\fs\ceph\export.c : 187 		req->r_path2 = kmalloc(16, GFP_NOFS);