basic: make check-pointer-deref clean

Add the needed assert changes to make the code clean
for the new check-pointer-deref script.

Note that for `_cmp()` functions its a bit of a question
if we want `assert()` or `POINTER_MAY_BE_NULL()` here. The
former is more correct but it adds a (small) runtime hit. If
we use POINTER_MAY_BE_NULL() it is compiled out but strictly
speaking it is wrong (so at least we would need a comment).

src/basic/string-util.c

@@ -174,6 +174,9 @@ char* ascii_strlower_n(char *s, size_t n) {
 
 int ascii_strcasecmp_n(const char *a, const char *b, size_t n) {
 
+        assert(a);
+        assert(b);
+
         for (; n > 0; a++, b++, n--) {
                 int x, y;
 
@@ -276,6 +279,10 @@ static bool string_has_ansi_sequence(const char *s, size_t len) {
 }
 
 static size_t previous_ansi_sequence(const char *s, size_t length, const char **ret_where) {
+
+        assert(s);
+        assert(ret_where);
+
         /* Locate the previous ANSI sequence and save its start in *ret_where and return length. */
 
         for (size_t i = length - 2; i > 0; i--) {  /* -2 because at least two bytes are needed */
@@ -1133,6 +1140,7 @@ int string_truncate_lines(const char *s, size_t n_lines, char **ret) {
         size_t n = 0;
 
         assert(s);
+        assert(ret);
 
         /* Truncate after the specified number of lines. Returns > 0 if a truncation was applied or == 0 if
          * there were fewer lines in the string anyway. Trailing newlines on input are ignored, and not
@@ -1187,6 +1195,8 @@ int string_extract_line(const char *s, size_t i, char **ret) {
         const char *p = s;
         size_t c = 0;
 
+        assert(ret);
+
         /* Extract the i'nth line from the specified string. Returns > 0 if there are more lines after that,
          * and == 0 if we are looking at the last line or already beyond the last line. As special
          * optimization, if the first line is requested and the string only consists of one line we return