libutils: fix overflow in String8::allocFromUTF8

Patch contributed in:
https://code.google.com/p/android/issues/detail?id=182908

Bug: 23290056

(cherry picked from commit 4eeacbeec0)

Change-Id: Ife1dc0791040150132bea6884f1e6c8d31972d1b

libutils/String8.cpp

@@ -78,6 +78,9 @@ void terminate_string8()
 static char* allocFromUTF8(const char* in, size_t len)
 {
     if (len > 0) {
+        if (len == SIZE_MAX) {
+            return NULL;
+        }
         SharedBuffer* buf = SharedBuffer::alloc(len+1);
         ALOG_ASSERT(buf, "Unable to allocate shared buffer");
         if (buf) {