Add xmalloc.h

Add header file for the quasi-standard [1] xmalloc call.

Include x-version of strndup. Other functions (e.g. xrealloc) can be
added as needed.

The current approach implemented in mx_util is to wait and retry on
ENOMEM malloc failure. However, this overhead doesn't seem to be
justified, because it is difficult to imagine a case, where a malloc
would fail with ENOMEM at one time and a retry would succeed.

[1] https://www.gnu.org/software/libc/manual/html_node/Malloc-Examples.html

xmalloc.h

@@ -0,0 +1,26 @@
+#ifndef _XMALLOC_H
+#define _XMALLOC_H 1
+
+#include <stdio.h>
+#include <stdlib.h>
+
+__attribute__ ((noreturn, unused)) static void out_of_memory() {
+    fprintf(stderr,"out of memory\n");
+    abort();
+}
+
+__attribute__ ((unused)) static void *xmalloc(size_t size) {
+    void *ptr = malloc(size);
+    if (__builtin_expect(ptr == NULL, 0))
+        out_of_memory();
+    return(ptr);
+}
+
+__attribute__ ((unused)) static char *xstrndup(const char *s, size_t n) {
+    char *ptr = strndup(s, n);
+    if (__builtin_expect(ptr == NULL, 0))
+        out_of_memory();
+    return(ptr);
+}
+
+#endif