Drop the unneeded pointer casting

The standard says that "A pointer to void may be converted to or from a
pointer to any object type". So the casting is unneeded.

REF:
    * c11: http://port70.net/~nsz/c/c11/n1570.html#6.3.2.3p1
    * c99: http://port70.net/~nsz/c/c99/n1256.html

Closes GH-5916

Zend/zend_alloc.c

@@ -1236,7 +1236,7 @@ static zend_never_inline void *zend_mm_alloc_small_slow(zend_mm_heap *heap, uint
 #endif
 
 	/* return first element */
-	return (char*)bin;
+	return bin;
 }
 
 static zend_always_inline void *zend_mm_alloc_small(zend_mm_heap *heap, int bin_num ZEND_FILE_LINE_DC ZEND_FILE_LINE_ORIG_DC)
@@ -1253,7 +1253,7 @@ static zend_always_inline void *zend_mm_alloc_small(zend_mm_heap *heap, int bin_
 	if (EXPECTED(heap->free_slot[bin_num] != NULL)) {
 		zend_mm_free_slot *p = heap->free_slot[bin_num];
 		heap->free_slot[bin_num] = p->next_free_slot;
-		return (void*)p;
+		return p;
 	} else {
 		return zend_mm_alloc_small_slow(heap, bin_num ZEND_FILE_LINE_RELAY_CC ZEND_FILE_LINE_ORIG_RELAY_CC);
 	}