Fixes a regression from 753a92952: if cb was negative, read_bytes
overflowed, because the type was changed from int to size_t.
Also fixes incorrect timeout calculation: timeout_ms was 200, but
each iteration delayed for 10ms, and reduced timeout_ms by 1. This
made the effective timeout to be 2000ms.
Calloc function tries to allocate 4096 bytes.
If such memory is not available, it returns NULL pointer.
This exception was not handled in the code, causing a dirty crash.
A 0 length string is still a valid input and should be treated as such, a NULL return should be reserved for when errors occur during line editing or EOF is reached.
Merges https://github.com/espressif/esp-idf/pull/4926
