gdb/gdbtypes.h: Fix comparison of uninitialized values

When called with an array type of unknown dimensions,
is_scalar_type_recursive ended up comparing uninitialized values.

This was picked up by the following compiler warning:

  CXX    gdbtypes.o
/binutils-gdb/gdb/gdbtypes.c: In function int is_scalar_type_recursive(type*):
/binutils-gdb/gdb/gdbtypes.c:3670:38: warning: high_bound may be used uninitialized in this function [-Wmaybe-uninitialized]
 3670 |       return high_bound == low_bound && is_scalar_type_recursive (elt_type);
      | ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/binutils-gdb/gdb/gdbtypes.c:3670:38: warning: low_bound may be used uninitialized in this function [-Wmaybe-uninitialized]

This patch makes sure that when dealing with an array of unknown size
(or an array of more than 1 element), is_scalar_type_recursive returns
false.

gdb/ChangeLog:

	* gdbtypes.c (is_scalar_type_recursive): Prevent comparison
	between uninitialized values.

Change-Id: Ifc005ced166aa7a065fef3e652977bae67625bf4