fuzz: Fix FUZZ_malloc_rand() to return non-NULL for zero-size allocations

The FUZZ_malloc_rand() function was incorrectly always returning NULL for
zero-size allocations. The random offset generated by
FUZZ_dataProducer_int32Range() was not being added to the pointer variable,
causing the function to always return (void *)0.

tests/fuzz/fuzz_helpers.h

@@ -66,6 +66,7 @@ void* FUZZ_malloc(size_t size);
 /**
  * malloc except returns random pointer for zero sized data and FUZZ_ASSERT
  * that malloc doesn't fail.
+ * WARNING: Only free the returned pointer if size > 0!
  */
 void* FUZZ_malloc_rand(size_t size,  FUZZ_dataProducer_t *producer);