Fix calloc to return NULL on malloc failure

Ensure calloc returns NULL when malloc fails to allocate memory. This
change prevents potential undefined behavior when memory allocation
fails.

Author: visitorckw

lib/c.c

@@ -654,6 +654,9 @@ void *malloc(int size)
 void *calloc(int n, int size)
 {
     char *p = malloc(n * size);
+
+    if (!p)
+        return NULL;
     for (int i = 0; i < n * size; i++)
         p[i] = 0;
     return p;