dynapi: Don't use SDL_getenv; it might malloc before the app sets an allocator.

Use platform-specific code instead, so SDL's allocator never comes into play.

src/dynapi/SDL_dynapi.c

@@ -440,7 +440,16 @@ extern SDL_NORETURN void SDL_ExitProcess(int exitcode);
 
 static void SDL_InitDynamicAPILocked(void)
 {
-    char *libname = SDL_getenv_REAL(SDL_DYNAMIC_API_ENVVAR);
+    /* this can't use SDL_getenv_REAL, because it might allocate memory before the app can set their allocator */
+#if defined(WIN32) || defined(_WIN32) || defined(__CYGWIN__)
+    /* We've always used LoadLibraryA for this, so this has never worked with Unicode paths on Windows. Sorry. */
+    char envbuf[512];  /* overflows will just report as environment variable being unset, but LoadLibraryA has a MAX_PATH of 260 anyhow, apparently. */
+    const DWORD rc = GetEnvironmentVariableA(SDL_DYNAMIC_API_ENVVAR, envbuf, (DWORD) sizeof (envbuf));
+    char *libname = ((rc != 0) && (rc < sizeof (envbuf))) ? envbuf : NULL;
+#else
+    char *libname = getenv(SDL_DYNAMIC_API_ENVVAR);
+#endif
+
     SDL_DYNAPI_ENTRYFN entry = NULL; /* funcs from here by default. */
     SDL_bool use_internal = SDL_TRUE;