Fix regression 'info variables' does not show minimal symbols.

12615cba Add [-q] [-t TYPEREGEXP] [NAMEREGEXP] args to info [args|functions|locals|variables]
introduced a regression that minimal symbols were not listed anymore, due to a wrong
condition checking the absence of a type regexp in the loop scanning the minimal symbols.

Instead, before entering the loop scanning the minimal symbols, check that we
do not have a type regexp, as we will never match a minimal symbol with
this type regexp.

With the fix in this patch, for this part of the code, we basically go back
to the GDB 8.2 logic, with just the addition of
  && !treg.has_value ())
to 'enter' in the minsym case.
This should ensure that at least there is no regression compared to 8.2,
when not using the new type matching argument, as there was no treg in 8.2.

2018-11-20  Philippe Waroquiers  <philippe.waroquiers@skynet.be>

	* symtab.c (search_symbols): Properly check absence of type regexp
	before entering the loop scanning the minimal symbols.