Don't call decode_line_with_current_source from select_source_symtab

Don't call decode_line_with_current_source from select_source_symtab

select_source_symtab currently calls decode_line_with_current_source.
However, this function iterates over all program spaces, and so it is
possible that it will return a "main" from some other program space.

This patch changes select_source_symtab to simply use the symbol it
already found in the current program space.

gdb/ChangeLog
2019-10-02  Tom Tromey  <tromey@adacore.com>

	* source.c (select_source_symtab): Don't call
	decode_line_with_current_source.