alloc: Avoid calling memcpy with a NULL pointer

ncclRealloc() is used in the code also to perform the initial allocation.
This results in calling memcpy() with a NULL source pointer and 0 copy size.

Allegedly this seems harmless, but memcpy is declared to never accept NULL pointers.
Which could lead to undefined behaviour.

This issue was caught by running nccl-tests with NCCL built with UBSAN:
```
include/alloc.h:68:9: runtime error: null pointer passed as argument 2, which is declared to never be null
    #0 0x7f867cf80932 in ncclResult_t ncclRealloc<ncclProxyConnection*>(ncclProxyConnection***, unsigned long, unsigned long) include/alloc.h:68
    NVIDIA#1 0x7f867cf6c838 in ncclProxyNewConnection /home/ubuntu/nccl/src/proxy.cc:942
    NVIDIA#2 0x7f867cf74ba8 in proxyConnInit /home/ubuntu/nccl/src/proxy.cc:1261
    NVIDIA#3 0x7f867cf78021 in proxyProgressAsync /home/liran/nccl/src/proxy.cc:1318
    NVIDIA#4 0x7f867cf79ff0 in proxyServiceInitOp /home/liran/nccl/src/proxy.cc:1377
    NVIDIA#5 0x7f867cf7c052 in ncclProxyService(void*) /home/ubuntu/nccl/src/proxy.cc:1507
    NVIDIA#6 0x7f867c3c8608 in start_thread /build/glibc-2.31/nptl/pthread_create.c:477
    NVIDIA#7 0x7f867bfba132 in __clone (/lib/x86_64-linux-gnu/libc.so.6)
```

Signed-off-by: Liran Alon <[email protected]>

src/include/alloc.h

@@ -65,8 +65,12 @@ ncclResult_t ncclRealloc(T** ptr, size_t oldNelem, size_t nelem) {
     WARN("Failed to malloc %ld bytes", nelem*sizeof(T));
     return ncclSystemError;
   }
-  memcpy(p, oldp, oldNelem*sizeof(T));
-  free(oldp);
+
+  if (oldp != NULL) {
+    memcpy(p, oldp, oldNelem*sizeof(T));
+    free(oldp);
+  }
+
   memset(p+oldNelem, 0, (nelem-oldNelem)*sizeof(T));
   *ptr = (T*)p;
   INFO(NCCL_ALLOC, "Mem Realloc old size %ld, new size %ld pointer %p", oldNelem*sizeof(T), nelem*sizeof(T), *ptr);