gdb: replace some calls to internal_error with gdb_assert

gdb: replace some calls to internal_error with gdb_assert

There are a few spots using the pattern:

  if (condition)
    internal_error (__FILE__, __LINE__,
		    _("failed internal consistency check"));

The message brings no value, since it's pretty the description of a
failed assertion.  Replace a few of these that are obvious with
gdb_assert.

gdb/ChangeLog:

	* exec.c (build_section_table): Replace internal_error with
	gdb_assert.
	(section_table_xfer_memory_partial): Likewise.
	* mdebugread.c (parse_partial_symbols): Likewise.
	* psymtab.c (lookup_partial_symbol): Likewise.
	* utils.c (wrap_here): Likewise.