Revert 5c593fae42 (xCalloc)

calloc() allows 'nmemb' or 'size' to be zero, in which case NULL may be
returned. Letting htop die because of either argument being zero doesn't
make sense.

As a side note: As size_t is unsigned, compiler should be able to optimize
conditional (nmemb > 0 && size > 0) to (nmemb && size). This theorically
shouldn't increase code size too much.

XAlloc.c

@@ -29,7 +29,7 @@ void* xMalloc(size_t size) {
 
 void* xCalloc(size_t nmemb, size_t size) {
    void* data = calloc(nmemb, size);
-   if (!data) {
+   if (!data && nmemb > 0 && size > 0) {
       fail();
    }
    return data;