[gdb/symtab] Fix uninitialized memory in buildsym_compunit::finish_block_internal

[gdb/symtab] Fix uninitialized memory in buildsym_compunit::finish_block_internal

When running test-case gdb.dwarf2/per-bfd-sharing.exp with target board stabs,
gdb either segfaults or asserts due to reading uninitialized memory, allocated
here in buildsym_compunit::finish_block_internal:
...
	      ftype->set_fields
		((struct field *)
		 TYPE_ALLOC (ftype, nparams * sizeof (struct field)));
...

Fix this by using TYPE_ZALLOC instead.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
PR symtab/30810
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=30810