io: read_fd should only report EOF on a zero-byte return value.

POSIX says that's the EOF indicator, not a non-zero a short read.

src/io/SDL_iostream.c

@@ -431,7 +431,7 @@ static size_t SDLCALL fd_read(void *userdata, void *ptr, size_t size, SDL_IOStat
             SDL_SetError("Error reading from datastream: %s", strerror(errno));
         }
         bytes = 0;
-    } else if (bytes < size) {
+    } else if (bytes == 0) {
         *status = SDL_IO_STATUS_EOF;
     }
     return (size_t)bytes;