Avoid zero-length calloc (#3280)

Using `calloc` function with a count of zero elements is implementation
defined and it may cause allocation errors on some platforms (ref #3277).
This commit fixes `jv_mem_calloc` callers to avoid undefined behavior.

src/jv_alloc.h

@@ -2,7 +2,6 @@
 #define JV_ALLOC_H
 
 #include <stddef.h>
-#include "jv.h"
 
 void* jv_mem_alloc(size_t);
 void* jv_mem_alloc_unguarded(size_t);