implement our own assert() function

I recently upgraded the software on my macOS server and was dismayed to
see that cppcheck reported a huge number of format string errors due to
mismatches between the format string and its arguments from calls to
`assert()`. It turns out they are due to the macOS header using `%lu`
for the line number which is obviously wrong since it is using the C
preprocessor `__LINE__` symbol which evaluates to a signed int.

I also noticed that the macOS implementation writes to stdout, rather
than stderr. It also uses `printf()` which can be a problem on some
platforms if the stream is already in wide mode which is the normal case
for fish.

So implement our own `assert()` implementation. This also eliminates
double-negative warnings that we get from some of our calls to
`assert()` on some platforms by oclint.

Also reimplement the `DIE()` macro in terms of our internal
implementation.

Rewrite `assert(0 && msg)` statements to `DIE(msg)` for clarity and to
eliminate oclint warnings about constant expressions.

Fixes #3276, albeit not in the fashion I originally envisioned.

src/builtin_string.cpp

@@ -5,7 +5,6 @@
 #ifdef _WIN32
 #define PCRE2_STATIC
 #endif
-#include <assert.h>
 #include <errno.h>
 #include <limits.h>
 #include <stdarg.h>
@@ -370,9 +369,7 @@ struct compiled_regex_t {
         }
 
         match = pcre2_match_data_create_from_pattern(code, 0);
-        if (match == 0) {
-            DIE_MEM();
-        }
+        assert(match);
     }
 
     ~compiled_regex_t() {
@@ -705,9 +702,8 @@ bool regex_replacer_t::replace_matches(const wchar_t *arg) {
 
     bool done = false;
     while (!done) {
-        if (output == NULL) {
-            DIE_MEM();
-        }
+        assert(output);
+
         PCRE2_SIZE outlen = bufsize;
         pcre2_rc = pcre2_substitute(regex.code, PCRE2_SPTR(arg), arglen,
                                     0,  // start offset