NUL-terminate the input buffer by assignment rather than using memset()

Writing a final NUL-terminator is sufficient rather than zeroing out the
unused slice of the input buffer. Suppresses a false-positive reported
by Coverity Scan.

Coverity CID 163308; Severity: Insignificant

pick.c

@@ -222,7 +222,7 @@ get_choices(void)
 			err(1, NULL);
 		size *= 2;
 	}
-	memset(buf + length, '\0', size - length);
+	buf[length] = '\0';
 
 	choices.size = 16;
 	if ((choices.v = reallocarray(NULL, choices.size,