Q: Given the following code slice:
```
1 static xmlChar *
2 xmlParseAttValueComplex(xmlParserCtxtPtr ctxt, int *attlen, int normalize) {
6     size_t len = 0;
8     int c, l, in_space = 0;
10     xmlEntityPtr ent;
12     if (NXT(0) == '"') {
13 	ctxt->instate = XML_PARSER_ATTRIBUTE_VALUE;
14 	limit = '"';
16     } else if (NXT(0) == '\'') {
17 	limit = '\'';
18 	ctxt->instate = XML_PARSER_ATTRIBUTE_VALUE;
28     buf_size = XML_PARSER_BUFFER_SIZE;
29     buf = (xmlChar *) xmlMallocAtomic(buf_size);
30     if (buf == NULL) goto mem_error;
35     c = CUR_CHAR(l);
36     while (((NXT(0) != limit) && /* checked */
43         if ((len > XML_MAX_TEXT_LENGTH) &&
49 	if (c == 0) break;
50 	if (c == '&') {
51 	    in_space = 0;
52 	    if (NXT(1) == '#') {
53 		int val = xmlParseCharRef(ctxt);
55 		if (val == '&') {
56 		    if (ctxt->replaceEntities) {
60 			buf[len++] = '&';
69 			buf[len++] = '&';
70 			buf[len++] = '#';
71 			buf[len++] = '3';
72 			buf[len++] = '8';
73 			buf[len++] = ';';
75 		} else if (val != 0) {
79 		    len += xmlCopyChar(0, &buf[len], val);
82 		ent = xmlParseEntityRef(ctxt);
83 		ctxt->nbentities++;
84 		if (ent != NULL)
85 		    ctxt->nbentities += ent->owner;
86 		if ((ent != NULL) &&
91 		    if ((ctxt->replaceEntities == 0) &&
93 			buf[len++] = '&';
94 			buf[len++] = '#';
95 			buf[len++] = '3';
96 			buf[len++] = '8';
97 			buf[len++] = ';';
99 			buf[len++] = ent->content[0];
101 		} else if ((ent != NULL) &&
103 		    if (ent->etype != XML_INTERNAL_PREDEFINED_ENTITY) {
104 			rep = xmlStringDecodeEntities(ctxt, ent->content,
107 			if (rep != NULL) {
108 			    current = rep;
109 			    while (*current != 0) { /* non input consuming */
110                                 if ((*current == 0xD) || (*current == 0xA) ||
112                                     buf[len++] = 0x20;
113                                     current++;
115                                     buf[len++] = *current++;
127 			if (ent->content != NULL)
128 			    buf[len++] = ent->content[0];
130 		} else if (ent != NULL) {
131 		    int i = xmlStrlen(ent->name);
132 		    const xmlChar *cur = ent->name;
151 		    buf[len++] = '&';
155 		    for (;i > 0;i--)
156 			buf[len++] = *cur++;
157 		    buf[len++] = ';';
161 	    if ((c == 0x20) || (c == 0xD) || (c == 0xA) || (c == 0x9)) {
162 	        if ((len != 0) || (!normalize)) {
169 		    in_space = 1;
172 	        in_space = 0;
181 	c = CUR_CHAR(l);
183     if (ctxt->instate == XML_PARSER_EOF)
186     if ((in_space) && (normalize)) {
187         while (buf[len - 1] == 0x20) len--;```
which has a CWE-125 vulnerability at line:
```
187         while (buf[len - 1] == 0x20) len--;
```
Please analyze the root cause of the vulnerability.