Fix regression in text_handler_multiselect in the text frontend

In commit 299ecb85, we started to use
readline to prompt the user, and as a result, the `answer` variable is
now dynamically allocated (before, it used to be a 4096 chars array on
the stack).

This broke two things.

First, the use of `sizeof(answer)` is not valid anymore, as it just
returns the size of a pointer, not the size of the memory area.

Second, even if we fix the above to get the size right, we have another
problem. There's no guarantee that `answer` is big enough to accomodate
the final answer. If we take the example of the d-i tasksel screen, the
answer from user is likely to be something like `1 7 12`, but then it's
converted to values, something like `Debian desktop environment, ...
MATE, standard system utilities`, which is much longer. So if we want to
reuse `answer` to hold the final answer, we'll need to reallocate it.

Therefore, with this commit, we do just that. We first run a loop to
compute the size needed for the answer, then we free/alloc answer for
this size, and we can finally use it.

Closes: #1025133

src/modules/frontend/text/text.c

@@ -620,6 +620,7 @@ static int text_handler_multiselect(struct frontend *obj, unsigned printed, stru
 	char **defaults;
 	char *defval, *cp;
 	char *answer = NULL;
+	size_t answer_len;
 	int i, j, dcount, choice;
 	int ret = DC_OK;
 	unsigned start = 0;
@@ -713,14 +714,24 @@ static int text_handler_multiselect(struct frontend *obj, unsigned printed, stru
 		}
 	}
 
-	answer[0] = 0;
+	answer_len = 0;
+	for (i = 0; i < choices->count; i++)
+	{
+		if (choices->selected[i])
+			answer_len += strlen(choices->choices[i]) + 2;
+	}
+
+	free(answer);
+	answer = malloc(answer_len);
+	if (answer_len > 0)
+		answer[0] = 0;
 	for (i = 0; i < choices->count; i++)
 	{
 		if (choices->selected[i])
 		{
 			if (answer[0] != 0)
-				strvacat(answer, sizeof(answer), ", ", NULL);
-			strvacat(answer, sizeof(answer), choices->choices[i], NULL);
+				strvacat(answer, answer_len, ", ", NULL);
+			strvacat(answer, answer_len, choices->choices[i], NULL);
 		}
 	}
 	question_setvalue(q, answer);