kex.c: kex_agree_instr() improve string reading (#552)
kex.c: kex_agree_instr() improve string reading (#552)

* kex.c: kex_agree_instr() improve string reading

file: kex.c
notes: if haystack isn't null terminated we should use memchr() not strchar(). We should also make sure we don't walk off the end of the buffer.
credit: 
Will Cosgrove, reviewed by Michael Buckley
libssh2/libssh2,libssh2/libssh2,libssh2/libssh2,libssh2/libssh2