instrument-c.adb: avoid uninitialized record component

Make sure that the record component PP_Info.PP_Source_Range is always
initialized, at least so that we do not stream absurd sloc range
information to SID/checkpoint files.

That went unnoticed so far because Natural'Write will happily write a
negative number, and Natural'Read will happily read and assign it,
without raising a Constraint_Error.

This fixes no known gnatcov issue as of today, but is necessary for the
upcoming work to make SID/checkpoint reading manual.

Author: pmderodat

tools/gnatcov/instrument-c.adb

@@ -735,6 +735,7 @@ package body Instrument.C is
       --  a Local_Source_Location as it can refer to #included files.
 
       Info.Actual_Source_Range := (From.L, To.L);
+      Info.PP_Source_Range := No_Local_Range;
 
       --  Check if this is comes from a macro expansion, in which case we need
       --  to record some information, for reporting purposes.