MINOR: cfgparse: load_cfg_in_mem: fix null ptr dereference reported by coverity

MINOR: cfgparse: load_cfg_in_mem: fix null ptr dereference reported by coverity

This helps to optimize a bit load_cfg_in_mem() and fixes the potential null ptr
dereference in fread() call. If (read_bytes + bytes_to_read) equals to initial
chunk_size (zero), realloc is never called, *cfg_content keeps its NULL value.

So, let's assure that initial number of bytes to read
(read_bytes + bytes_to_read) is stricly positive, when we enter into loop at
the first time.