iconv: Fix memory management in autoconf test for //IGNORE

Sometimes running `./configure` for ASAN hangs on my machine. This is
reproducible when compiling and running the program manually using

    clang-16 -g -fsanitize=undefined,address test.c

It seems this is related to the LLVM symbolizer getting stuck, and the
symbolizer is likely required to report the memory leak. After adding the
memory management functions it seems to work (or rather fail) reliably.

Author: TimWolla

ext/iconv/config.m4

@@ -112,6 +112,8 @@ int main(void) {
   char *out = malloc(out_left);
   char *out_p = out;
   size_t result = iconv(cd, (char **) &in_p, &in_left, (char **) &out_p, &out_left);
+  free(out);
+  iconv_close(cd);
   if(result == (size_t)-1) {
     return 1;
   }