Use ZendMM for libgmp

As is, ext/gmp uses the default memory allocator of the underlying
libgmp; this allocator is fallible, but contrary to ZendMM, it
`abort()`s without giving us an opportunity to do a cleaner shutdown.
Furthermore, the libgmp allocator obviously doesn't heed memory_limit.
Thus we install wrappers of the ZendMM API as libgmp allocator.

Author: cmb69

ext/gmp/gmp.c

@@ -521,6 +521,21 @@ static int gmp_unserialize(zval *object, zend_class_entry *ce, const unsigned ch
 }
 /* }}} */
 
+static void *gmp_alloc(size_t size)
+{
+	return emalloc(size);
+}
+
+static void *gmp_realloc(void *ptr, size_t old_size, size_t new_size)
+{
+	return erealloc(ptr, new_size);
+}
+
+static void gmp_free(void *ptr, size_t size)
+{
+	efree(ptr);
+}
+
 /* {{{ ZEND_GINIT_FUNCTION */
 static ZEND_GINIT_FUNCTION(gmp)
 {
@@ -551,6 +566,8 @@ ZEND_MINIT_FUNCTION(gmp)
 
 	register_gmp_symbols(module_number);
 
+	mp_set_memory_functions(gmp_alloc, gmp_realloc, gmp_free);
+
 	return SUCCESS;
 }
 /* }}} */