Add xallocarray() helper macro

Uses reallocarray to perform integer overflow detection when allocating
an array, using NULL as the previous pointer to force a new allocation.

Signed-off-by: Alan Coopersmith <alan.coopersmith@oracle.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>

include/os.h

@@ -74,6 +74,8 @@ typedef struct _NewClientRec *NewClientPtr;
 
 #define xstrdup(s) Xstrdup(s)
 #define xnfstrdup(s) XNFstrdup(s)
+
+#define xallocarray(num, size) reallocarray(NULL, (num), (size))
 #endif
 
 #include <stdio.h>