ptrace_write_string: make sure we always write the terminating NUL.

We can't check *str for NUL since it may not have been written yet.

src/exec_ptrace.c

@@ -465,8 +465,10 @@ ptrace_write_string(pid_t pid, long addr, const char *str)
 		__func__, (int)pid, addr, (int)sizeof(u.buf), u.buf);
 	    debug_return_size_t(-1);
 	}
-	if (*str == '\0')
+	if ((u.word & 0xff) == 0) {
+	    /* If the last byte we wrote is a NUL we are done. */
 	    debug_return_size_t(str - str0 + 1);
+	}
 	addr += sizeof(long);
     }
 }