fix(client): Clear internal client buffer on failed packet send

[TreeHunter9]

Previously a failed send left stale bytes in the buffer, which got prepended to the next packet.

[TreeHunter9]

The incorrect behavior can be seeing in this example:

#include "ibase.h"
#include <stdio.h>
#include <string.h>

static void print_status_vector(const ISC_STATUS *status_vector) {
	char buffer[512] = {};
	while (fb_interpret(buffer, 512, &status_vector))
		printf("%s\n", buffer);
}

int main(int argc, char **argv) {
	const char *db  = "localhost:employee", 
			   *usr = "sysdba",
			   *pwd = "masterkey";

	ISC_STATUS_ARRAY st = {0};
	isc_db_handle dbh = 0;
	isc_tr_handle tr = 0;
	char dpb[256]; int dl = 0;
	dpb[dl++] = isc_dpb_version1;
	dpb[dl++] = isc_dpb_user_name;
	dpb[dl++] = (char)strlen(usr);
	memcpy(dpb + dl, usr, strlen(usr));
	dl += strlen(usr);
	dpb[dl++] = isc_dpb_password;
	dpb[dl++] = (char)strlen(pwd);
	memcpy(dpb + dl, pwd, strlen(pwd));
	dl += strlen(pwd);

	if (isc_attach_database(st, strlen(db), (char *)db, &dbh, dl, dpb)) {
		print_status_vector(st);
		return 1;
	}
	if (isc_start_transaction(st, &tr, 1, &dbh, 0, NULL)) {
		print_status_vector(st);
		return 1;
	}

	ISC_ARRAY_DESC desc;
	desc.array_desc_dtype = 8;
	desc.array_desc_scale = 0;
	desc.array_desc_length = 4;
	memcpy(desc.array_desc_field_name, "A", 1);
	memcpy(desc.array_desc_relation_name, "ARR", 3);
	desc.array_desc_dimensions = 1;
	desc.array_desc_flags = 0;
	desc.array_desc_bounds[0].array_bound_lower = 1;
	desc.array_desc_bounds[0].array_bound_upper = 3;

	ISC_QUAD aid = {0, 1};

	unsigned char sdl[64]; int n = 0;
	sdl[n++] = 2;

	char slice[256] = {0}; ISC_LONG slen = 0;
	// Will throw an error in the client lib, packet will not be send, but it is stored in buffer.
	isc_get_slice(st, &dbh, &tr, &aid, (unsigned short)n, (const ISC_UCHAR *)sdl,
				0, NULL, (ISC_LONG)sizeof(slice), slice, &slen);
	print_status_vector(st);

	// Will send two packets, incomplete `isc_get_slice` and `isc_commit_transaction`.
	// The client will get an error and will think that it is from `isc_commit_transaction`, but it actually from `isc_get_slice` API call.
	isc_commit_transaction(st, &tr);
	print_status_vector(st);
	isc_detach_database(st, &dbh);
	print_status_vector(st);
	return 0;
}