mingw: be more defensive when making the environment block

Outside of our Windows-specific code, the end of the environment can be
marked also by a pointer to a NUL character, not only by a NULL pointer
as our code assumed so far.

That led to a buffer overrun in `make_environment_block()` when running
`git-remote-https` in `mintty` (because `curl_global_init()` added the
`CHARSET` environment variable *outside* of `mingw_putenv()`, ending the
environment in a pointer to an empty string).

Side note for future debugging on Windows: when running programs in
`mintty`, the standard input/output/error is not connected to a Win32
Console, but instead is pipe()d. That means that even stderr may not be
written completely before a crash, but has to be fflush()ed explicitly.
For example, when debugging crashes, the developer should insert an
`fflush(stderr);` at the end of the `error()` function defined in
usage.c.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

compat/mingw.c

@@ -977,19 +977,19 @@ static wchar_t *make_environment_block(char **deltaenv)
 	char **tmpenv;
 	int i = 0, size = environ_size, wenvsz = 0, wenvpos = 0;
 
-	while (deltaenv && deltaenv[i])
+	while (deltaenv && deltaenv[i] && *deltaenv[i])
 		i++;
 
 	/* copy the environment, leaving space for changes */
 	tmpenv = xmalloc((size + i) * sizeof(char*));
 	memcpy(tmpenv, environ, size * sizeof(char*));
 
 	/* merge supplied environment changes into the temporary environment */
-	for (i = 0; deltaenv && deltaenv[i]; i++)
+	for (i = 0; deltaenv && deltaenv[i] && *deltaenv[i]; i++)
 		size = do_putenv(tmpenv, deltaenv[i], size, 0);
 
 	/* create environment block from temporary environment */
-	for (i = 0; tmpenv[i]; i++) {
+	for (i = 0; tmpenv[i] && *tmpenv[i]; i++) {
 		size = 2 * strlen(tmpenv[i]) + 2; /* +2 for final \0 */
 		ALLOC_GROW(wenvblk, (wenvpos + size) * sizeof(wchar_t), wenvsz);
 		wenvpos += xutftowcs(&wenvblk[wenvpos], tmpenv[i], size) + 1;