MXS-2507: Check stored integer size

If the avro block is smaller than the size of the stored data, the file
was created with a block size that was too small. Even the reference Avro
implementation can't read the file in this case.

avro/maxavro.c

@@ -142,6 +142,8 @@ char* maxavro_read_string(MAXAVRO_FILE* file, size_t* size)
     uint64_t len;
 
     if (maxavro_read_integer(file, &len))
+    {
+        if (file->buffer_ptr + len < file->buffer_end)
         {
             key = MXS_MALLOC(len + 1);
             if (key)
@@ -156,6 +158,11 @@ char* maxavro_read_string(MAXAVRO_FILE* file, size_t* size)
                 file->last_error = MAXAVRO_ERR_MEMORY;
             }
         }
+        else
+        {
+            file->last_error = MAXAVRO_ERR_MEMORY;
+        }
+    }
     return key;
 }