fix realloc with 0 mem size

--- debug log ---
```
sending request to "spain.ucoddcd.xyz"
request "+<[vector|addition|5,5]{1.000000,2.000000,3.000000,4.000000,5.000000,1.000000,0.000000,-1.000000,-2.000000,-3.000000}>"115
resizing response to 0
[1]    5271 segmentation fault  ./test.out
```