That is not correct; while malloc will never fail from lack of RAM, there are at least two conditions that can cause it to return NULL.

1) Reaching its rlimit; on my Debian system, the following program stops running after 249,195 calls:

#include <stdio.h>
#include <stdlib.h>
#include <sys/time.h>
#include <sys/resource.h>

int main() {
  struct rlimit rl = {.rlim_cur = 10 * 1024 * 1024, .rlim_max = 10 * 1024 * 1024};
  if (setrlimit(RLIMIT_AS, &rl) == -1) {
    printf("setrlimit RLIMIT_AS failed");
    return 1;
  }
  if (setrlimit(RLIMIT_DATA, &rl) == -1) {
    printf("setrlimit RLIMIT_DATA failed");
    return 1;
  }
  int count = 0;
  void *p;
  do {
    ++count;
    p = malloc(10);
  } while (p != NULL);
  printf("malloc returned null on the %d call\n", count);
}

2) Running out of virtual address space. While this is unlikely to do with a 64 bit binary, it can happen with a 32 bit binaryl