journal-remote : Ensure reallocation of source->buf does not result in source->size...

journal-remote : Ensure reallocation of source->buf does not result in source->size < source->filled (#3086)

While the function journal-remote-parse.c:get_line() enforces an assertion that source->filled <= source->size, in function journal-remote-parse.c:process_source() there is a chance that source->size will be decreased to a lower value than source->filled, when source->buf is reallocated. Therefore a check is added that ensures that source->buf is reallocated only when source->filled is smaller than target / 2.