Fix the memory leak in blob_buf

There were some places where blob_buf was allocated but never freed.
Valgrind found it in the following backtrace:

==18922== 166,400 bytes in 650 blocks are definitely lost in loss record 35 of 38
==18922==    at 0x488FDE4: realloc (vg_replace_malloc.c:1649)
==18922==    by 0x5E861A3: blob_buffer_grow (blob.c:26)
==18922==    by 0x5E86213: blob_buf_grow (blob.c:63)
==18922==    by 0x5E862CF: blob_add (blob.c:78)
==18922==    by 0x5E8633F: blob_buf_init (blob.c:97)
==18922==    by 0x5F5068F: ??? (in /usr/lib/asterisk/modules/chan_voicemngr.so)