Assert allocating non-zero size memory

Allocating zero size memory results in implementation-defined behavior:

  man:malloc(3) :
    If size is 0, then malloc() returns either NULL, or a unique pointer
    value that can later be successfully passed to free().

Meter.c

@@ -38,7 +38,7 @@ Meter* Meter_new(struct ProcessList_* pl, int param, const MeterClass* type) {
    this->param = param;
    this->pl = pl;
    this->curItems = type->maxItems;
-   this->values = xCalloc(type->maxItems, sizeof(double));
+   this->values = type->maxItems ? xCalloc(type->maxItems, sizeof(double)) : NULL;
    this->total = type->total;
    this->caption = xStrdup(type->caption);
    if (Meter_initFn(this))

XUtils.c

@@ -25,26 +25,26 @@ void fail() {
 }
 
 void* xMalloc(size_t size) {
+   assert(size > 0);
    void* data = malloc(size);
-   if (!data && size > 0) {
+   if (!data) {
       fail();
    }
    return data;
 }
 
 void* xCalloc(size_t nmemb, size_t size) {
+   assert(nmemb > 0);
+   assert(size > 0);
    void* data = calloc(nmemb, size);
-   if (!data && nmemb > 0 && size > 0) {
+   if (!data) {
       fail();
    }
    return data;
 }
 
 void* xRealloc(void* ptr, size_t size) {
-   if (!size) {
+   assert(size > 0);
-      free(ptr);
-      return NULL;
-   }
    void* data = realloc(ptr, size); // deepcode ignore MemoryLeakOnRealloc: this goes to fail()
    if (!data) {
       free(ptr);