esp8266/posix_helpers: Set ENOMEM on memory alloc failure.

POSIX requires malloc(), etc. to set ENOMEM on the failure, and e.g.
BerkeleyDB relies on this:

http://pubs.opengroup.org/onlinepubs/9699919799/functions/malloc.html

This should fix confusing OSError exceptions with 0 error code when
working with btree module.

ports/esp8266/posix_helpers.c

@@ -26,22 +26,33 @@
 
 #include <stdint.h>
 #include <stdio.h>
+#include <errno.h>
 #include "py/mphal.h"
 #include "py/gc.h"
 
 // Functions for external libs like axTLS, BerkeleyDB, etc.
 
 void *malloc(size_t size) {
-    return gc_alloc(size, false);
+    void *p = gc_alloc(size, false);
+    if (p == NULL) {
+        // POSIX requires ENOMEM to be set in case of error
+        errno = ENOMEM;
+    }
+    return p;
 }
 void free(void *ptr) {
     gc_free(ptr);
 }
 void *calloc(size_t nmemb, size_t size) {
-    return m_malloc0(nmemb * size);
+    return malloc(nmemb * size);
 }
 void *realloc(void *ptr, size_t size) {
-    return gc_realloc(ptr, size, true);
+    void *p = gc_realloc(ptr, size, true);
+    if (p == NULL) {
+        // POSIX requires ENOMEM to be set in case of error
+        errno = ENOMEM;
+    }
+    return p;
 }
 
 #define PLATFORM_HTONL(_n) ((uint32_t)( (((_n) & 0xff) << 24) | (((_n) & 0xff00) << 8) | (((_n) >> 8)  & 0xff00) | (((_n) >> 24) & 0xff) ))