Fix a crash in externals when in-memory needs to be reinterpolated. (#…

…8406)

Fixes #5715 but this unveiled other bugs in grdblend that will be addressed later.

src/grdblend.c

@@ -502,8 +502,9 @@ GMT_LOCAL int grdblend_init_blend_job (struct GMT_CTRL *GMT, char **files, unsig
  return (-GMT_RUNTIME_ERROR);
  t = NULL; /* To remind us that this is now gone */
  if ((B[n].G = GMT_Read_Data (GMT->parent, GMT_IS_GRID, GMT_IS_FILE, GMT_IS_SURFACE, GMT_CONTAINER_ONLY|GMT_GRID_ROW_BY_ROW, NULL, B[n].file, NULL)) == NULL) {
- return (-GMT_DATA_READ_ERROR);
+ return -GMT_DATA_READ_ERROR;
  }
+ B[n].memory = false; /* Since we read from file, even if it was a mem-grid before */
  t = B[n].G->header; /* Since it got reallocated */
  if (grdblend_overlap_check (GMT, &B[n], h, 0)) continue; /* In case grdconvert changed the region */
  }