Add test for further MPICH fail.

* See https://github.com/pmodels/mpich/commit/62f750cca3.

config/checkdoc/test_mpi_pack_external_3.txt

+test_mpi_pack_external_1.txt
\ No newline at end of file

config/checksrc/test_mpi_pack_external_3.c

+#include <stdbool.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <mpi.h>
+
+static const char datarep[] = "external32";
+
+enum { hdr_words = 2, dta_words = 12 };
+
+#ifdef NO_DETECT_TRAILING_CORRUPTION
+/* used to investigate with valgrind */
+enum { trailing_size = 0 };
+#else
+enum { trailing_size = 8 };
+#endif
+
+int main(int argc, char **argv)
+{
+  unsigned header[hdr_words];
+  const double dbl_canary = 0.356789;
+  size_t dta_words_extra = trailing_size > 0;
+  double *data;
+  size_t data_chars = sizeof data[0] * (dta_words + dta_words_extra);
+  data = malloc(data_chars);
+  if (!data) {
+    fprintf(stderr, "memory allocation for %zu bytes failed!\n", data_chars);
+    MPI_Abort(MPI_COMM_WORLD, 1);
+  }
+  MPI_Aint header_size, data_size;
+  MPI_Init(&argc, &argv);
+  header[0] = 2;
+  header[1] = 3;
+  MPI_Pack_external_size(datarep, hdr_words, MPI_UNSIGNED, &header_size);
+  MPI_Pack_external_size(datarep, dta_words, MPI_DOUBLE, &data_size);
+  MPI_Aint buf_size = header_size + data_size;
+  for (size_t i = 0; i < dta_words; ++i)
+    data[i] = (double)i * 1.25;
+  unsigned char *buf = malloc((size_t)buf_size + trailing_size);
+  if (!buf) {
+    fprintf(stderr, "memory allocation for %zu bytes failed!\n",
+            (size_t)buf_size + trailing_size);
+    MPI_Abort(MPI_COMM_WORLD, 1);
+  }
+  for (size_t i = 0; i < trailing_size; ++i)
+    buf[buf_size+i] = (unsigned char)i;
+  {
+    MPI_Aint pos_pack = 0;
+    MPI_Pack_external(datarep, header, hdr_words, MPI_UNSIGNED,
+                      buf, buf_size, &pos_pack);
+    MPI_Pack_external(datarep, data, dta_words, MPI_DOUBLE,
+                      buf, buf_size, &pos_pack);
+  }
+  for (size_t i = 0; i < dta_words_extra; ++i)
+    data[dta_words+i] = dbl_canary;
+  bool mismatch = false;
+  for (size_t i = 0; i < trailing_size; ++i)
+    mismatch |= buf[buf_size+i] != (unsigned char)i;
+  if (mismatch) {
+    fputs("data corruption after external32 packing detected\n", stderr);
+    MPI_Abort(MPI_COMM_WORLD, 1);
+  }
+  {
+    MPI_Aint pos_unpack = 0;
+    MPI_Unpack_external(datarep, buf, 104, &pos_unpack,
+                        header, hdr_words, MPI_UNSIGNED);
+    MPI_Unpack_external(datarep, buf, 104, &pos_unpack,
+                        data, dta_words, MPI_DOUBLE);
+  }
+  mismatch = (header[0] != 2 || header[1] != 3);
+  for (size_t i = 0; i < dta_words; ++i)
+    mismatch |= (data[i] != (double)i * 1.25);
+  for (size_t i = 0; i < dta_words_extra; ++i)
+  {
+    fprintf(stderr, "data[%zu]=%f\n", dta_words+i, data[dta_words+i]);
+    mismatch |= (data[dta_words+i] != dbl_canary);
+  }
+  if (mismatch) {
+    fputs("data corruption after external32 unpacking detected\n", stderr);
+    MPI_Abort(MPI_COMM_WORLD, 1);
+  }
+  free(buf);
+  MPI_Finalize();
+  return 0;
+}