Fix malloc and realloc overflow

If user requests a size >= 65532, adding the heap admin size
overflows size. Fixes #2358.

test/val/lib_common_malloc.c

@@ -0,0 +1,34 @@
+#include <errno.h>
+#include <stdlib.h>
+#include <string.h>
+#include "unittest.h"
+
+TEST
+{
+    char *buf, *buf2;
+    unsigned int i;
+
+    buf = malloc(0);
+    ASSERT_IsTrue (buf == NULL, "malloc (0) returned something");
+
+    for (i = 1; i < 10; i++) {
+      buf = malloc(i);
+      ASSERT_IsTrue (buf != NULL, "small returned nothing");
+    }
+
+    buf = malloc(4096);
+    ASSERT_IsTrue (buf != NULL, "malloc (4096) returned nothing");
+
+    buf = malloc(61000UL);
+    ASSERT_IsTrue (buf == NULL, "malloc (61000) returned something");
+
+    for (i = 65535UL; i > _heapmaxavail(); i--) {
+      buf = malloc(i);
+      ASSERT_IsTrue (buf == NULL, "malloc returned something but shouldn't have");
+    }
+
+    buf = malloc(i);
+    ASSERT_IsTrue (buf != NULL, "malloc returned nothing but should have");
+    ASSERT_IsTrue(_heapmaxavail() == 0, "heapmaxavail should be 0");
+}
+ENDTEST