Check for null-terminator in ResStringPool::string8At

All other stringAt methods check for null termination. Be consistent
so that upper levels don't end up with huge corrupt strings.

Bug: 62537081
Test: none
Change-Id: I17bdfb0c1e34507b66c6cad651bbdb12c5d4c417
(cherry picked from commit 3d35a0ea30)
(cherry picked from commit 97f8cb01149b35b1832c7f9efe85ff19edf1083e)

libs/androidfw/ResourceTypes.cpp

@@ -785,7 +785,13 @@ const char* ResStringPool::string8At(size_t idx, size_t* outLen) const
             *outLen = decodeLength(&str);
             size_t encLen = decodeLength(&str);
             if ((uint32_t)(str+encLen-strings) < mStringPoolSize) {
-                return (const char*)str;
+                // Reject malformed (non null-terminated) strings
+                if (str[encLen] != 0x00) {
+                    ALOGW("Bad string block: string #%d is not null-terminated",
+                          (int)idx);
+                    return NULL;
+                }
+              return (const char*)str;
             } else {
                 ALOGW("Bad string block: string #%d extends to %d, past end at %d\n",
                         (int)idx, (int)(str+encLen-strings), (int)mStringPoolSize);