Implement calloc() (accidentally got dropped with the malloc rewrite).

lang/cem/libcc.ansi/malloc/calloc.c

@@ -0,0 +1,23 @@
+#include <stdlib.h>
+#include <stdint.h>
+#include <unistd.h>
+
+void* calloc(size_t nmemb, size_t size)
+{
+    size_t bytes = nmemb * size;
+    void* ptr;
+    
+    /* Test for overflow.
+     * See http://stackoverflow.com/questions/1815367/multiplication-of-large-numbers-how-to-catch-overflow
+     */
+
+    if ((nmemb == 0) || (size == 0) || (nmemb > (SIZE_MAX / size)))
+        return NULL;
+    
+    ptr = malloc(bytes);
+    if (!ptr)
+        return NULL;
+
+    memset(ptr, 0, bytes);
+    return ptr;
+}