Clean up '-Wall' warnings from GCC

[bbassett-tibco]

When building with -Wall using current versions of GCC (for example, when building spotfiresoftware/spotfire-python using GCC 10.2.1 from the python:3.11 Docker image), the SBDF code emits a number of warnings. Clean up these warnings to reduce the number of warnings emitted when including the sources in another build (where we don't control the compiler flags added).

• -Wparentheses: There are two forms of this warning (the former is the most common warning encountered):

src/columnmetadata.c: In function ‘sbdf_cm_get_type’:
src/columnmetadata.c:56:6: warning: suggest parentheses around assignment used as truth value [-Wparentheses]
   56 |  if (error = sbdf_md_get(SBDF_COLUMNMETADATA_DATATYPE, inp, &obj))
      |      ^~~~~
src/columnmetadata.c:61:103: warning: suggest parentheses around ‘&&’ within ‘||’ [-Wparentheses]
   61 |  if (obj->type.id != SBDF_BINARYTYPEID || obj->count != 1 || sbdf_ba_get_len(*(void**)obj->data) != 1 && sbdf_ba_get_len(*(void**)obj->data) != 3)
      |                                                              ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• -Wunused-function:

At top level:
src/internals.c:18:12: warning: ‘get_packed_digits_size’ defined but not used [-Wunused-function]
   18 | static int get_packed_digits_size(int digits)
      |            ^~~~~~~~~~~~~~~~~~~~~~

• -Wsign-compare:

src/object.c: In function ‘sbdf_write_objects’:
src/object.c:399:39: warning: comparison of integer expressions of different signedness: ‘size_t’ {aka ‘long unsigned int’} and ‘int’ [-Wsign-compare]
  399 |       if (fwrite(*data, 1, length, f) != length)
      |                                       ^~
src/object.c: In function ‘sbdf_obj_copy’:
src/object.c:486:37: warning: operand of ‘?:’ changes signedness from ‘int’ to ‘long unsigned int’ due to unsignedness of other operand [-Wsign-compare]
  486 |  elem_size = is_array?sizeof(void*):sbdf_get_unpacked_size(t->type);
      |                                     ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

• -Wincompatible-pointer-types:

src/tablemetadata.c: In function ‘sbdf_tm_write’:
src/tablemetadata.c:583:64: warning: passing argument 4 of ‘qsort’ from incompatible pointer type [-Wincompatible-pointer-types]
  583 |  qsort((void*)array, array_size, sizeof(struct metadata_sort), compare_metadata_sort_by_name);
      |                                                                ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                                                                |
      |                                                                int (*)(const struct metadata_sort *, const struct metadata_sort *)
In file included from vendor/sbdf-c/src/tablemetadata.c:8:
/usr/include/stdlib.h:831:20: note: expected ‘__compar_fn_t’ {aka ‘int (*)(const void *, const void *)’} but argument is of type ‘int (*)(const struct metadata_sort *, const struct metadata_sort *)’
  831 |      __compar_fn_t __compar) __nonnull ((1, 4));
      |      ~~~~~~~~~~~~~~^~~~~~~~

• -Wunused-but-set-variable:

src/valuearray.c: In function ‘sbdf_va_create_rle’:
src/valuearray.c:112:7: warning: variable ‘is_string’ set but not used [-Wunused-but-set-variable]
  112 |   int is_string;
      |       ^~~~~~~~~