Don't store null terminator in string buffer (#8)

We were not accounting for the null terminator in the string allocation,
meaning that every string copy overflowed its buffer by one byte.

Usually the null terminator ended up getting overwritten by the first
character of the next string copied into the buffer, but occasionally,
due to concurrency, the null terminator from the first string could
overwrite the first character of the second.

Since the null terminator is not actually necessary, this commit removes
it from the string buffer. Now the allocation size is correct.

Co-authored-by: Evan Shaw <[email protected]>

zend_tombs_strings.c

@@ -103,7 +103,6 @@ static zend_always_inline zend_tombs_string_t* zend_tombs_strings_copy(zend_stri
            ZSTR_VAL(string),
            ZSTR_LEN(string));
 
-    copy->value[ZSTR_LEN(string)] = 0;
     copy->hash = ZSTR_HASH(string);
 
     __atomic_store_n(&copy->length, ZSTR_LEN(string), __ATOMIC_SEQ_CST);