Help understanding some valgrind output

Here is my valgrind output that I don't understand:

==319842== Invalid write of size 1
==319842==    at 0x48436E4: mempcpy (in
/usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==319842==    by 0x50CD1D8: _IO_default_xsputn (genops.c:386)
==319842==    by 0x50CD1D8: _IO_default_xsputn (genops.c:370)
==319842==    by 0x50B227B: __vfprintf_internal (vfprintf-internal.c:1688)
==319842==    by 0x50C0278: __vsprintf_internal (iovsprintf.c:95)
==319842==    by 0x509D047: sprintf (sprintf.c:30)
==319842==    by 0x10B88F: html_link_find (crawler.c:452)
==319842==    by 0x10BD6F: html_parse (crawler.c:536)
==319842==    by 0x10C2CB: check_multi_info (crawler.c:678)
==319842==    by 0x10C3DA: event_cb (crawler.c:706)
==319842==    by 0x10D828: crawler_init (crawler.c:1154)
==319842==    by 0x10DAE8: main (crawler.c:1207)
==319842==  Address 0xf107d18 is 0 bytes after a block of size 8,200 alloc'd
==319842==    at 0x483B7F3: malloc (in
/usr/lib/x86_64-linux-gnu/valgrind/vgpreload_memcheck-amd64-linux.so)
==319842==    by 0x10B736: html_link_find (crawler.c:440)
==319842==    by 0x10BD6F: html_parse (crawler.c:536)
==319842==    by 0x10C2CB: check_multi_info (crawler.c:678)
==319842==    by 0x10C3DA: event_cb (crawler.c:706)
==319842==    by 0x10D828: crawler_init (crawler.c:1154)
==319842==    by 0x10DAE8: main (crawler.c:1207)
==319842==

valgrind: m_mallocfree.c:305 (get_bszB_as_is): Assertion 'bszB_lo ==
bszB_hi' failed.
valgrind: Heap block lo/hi size mismatch: lo = 8272, hi =
3625731377157460067.
This is probably caused by your program erroneously writing past the
end of a heap block and corrupting heap metadata.  If you fix any
invalid writes reported by Memcheck, this assertion failure will
probably go away.  Please try that before reporting this as a bug.

The code this pertains to can be found at
https://github.com/JamesRead5737/webcrawler

Any help in understanding what this error means would be greatly
appreciated.

James Read

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