Add just-to-be-sure check to allocate_field().

This check will help to detect bugs earlier, and is quite lightweight
compared to malloc() anyway.

pb_decode.c

@@ -482,11 +482,15 @@ static bool checkreturn allocate_field(pb_istream_t *stream, void *pData, size_t
 {    
     void *ptr = *(void**)pData;
     
+    if (data_size == 0 || array_size == 0)
+        PB_RETURN_ERROR(stream, "invalid size");
+    
     /* Check for multiplication overflows.
      * This code avoids the costly division if the sizes are small enough.
      * Multiplication is safe as long as only half of bits are set
      * in either multiplicand.
      */
+    {
         const size_t check_limit = (size_t)1 << (sizeof(size_t) * 4);
         if (data_size >= check_limit || array_size >= check_limit)
         {
@@ -496,6 +500,7 @@ static bool checkreturn allocate_field(pb_istream_t *stream, void *pData, size_t
                 PB_RETURN_ERROR(stream, "size too large");
             }
         }
+    }
     
     /* Allocate new or expand previous allocation */
     /* Note: on failure the old pointer will remain in the structure,