util-lib: [static] array argument sizes are apparently not OK for NULL parameters

Let's drop the 'static' logic when a parameter can be NULL.

I think asan/ubsan are right here, judging by the C99 spec language:

"A declaration of a parameter as ‘‘array of type’’ shall be adjusted to
‘‘qualified pointer to type’’, where the type qualifiers (if any) are
those specified within the [ and ] of the array type derivation. If the
keyword static also appears within the [ and ] of the array type
derivation, then for each call to the function, the value of the
corresponding actual argument shall provide access to the first element
of an array with at least as many elements as specified by the size
expression."

If we specify NULL, then we certainly don't pvode access to any valid
array.

Fixes: #13039

src/basic/string-util.c

@@ -725,10 +725,17 @@ char *strreplace(const char *text, const char *old_string, const char *new_strin
         return ret;
 }
 
-static void advance_offsets(ssize_t diff, size_t offsets[static 2], size_t shift[static 2], size_t size) {
+static void advance_offsets(
+                ssize_t diff,
+                size_t offsets[2], /* note: we can't use [static 2] here, since this may be NULL */
+                size_t shift[static 2],
+                size_t size) {
+
         if (!offsets)
                 return;
 
+        assert(shift);
+
         if ((size_t) diff < offsets[0])
                 shift[0] += size;
         if ((size_t) diff < offsets[1])
@@ -844,8 +851,7 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
 
         fclose(f);
 
-        free(*ibuf);
-        *ibuf = obuf;
+        free_and_replace(*ibuf, obuf);
 
         if (_isz)
                 *_isz = osz;
@@ -855,7 +861,7 @@ char *strip_tab_ansi(char **ibuf, size_t *_isz, size_t highlight[2]) {
                 highlight[1] += shift[1];
         }
 
-        return obuf;
+        return *ibuf;
 }
 
 char *strextend_with_separator(char **x, const char *separator, ...) {