Instrument.C: properly error out when C source could not be parsed

If a C source can't be parsed, the resulting translation unit will be
null. In this case, emit an error message and error out. gnatcov used to
crash on an assertion failure.

Change-Id: I4da2a21539ba23260f2308d6e9df8effd2bbff00
TN: V816-018

Author: Jugst3r

tools/gnatcov/instrument-c.adb

@@ -2547,6 +2547,13 @@ package body Instrument.C is
               Unsaved_Files         => null,
               Num_Unsaved_Files     => 0,
               Options               => 0);
+         if Self.TU = null then
+            Outputs.Error ("Failed to parse " & Filename);
+            Outputs.Error ("Please make sure that the original project can"
+                           & " be compiled, and that the right set of"
+                           & " options is passed to gnatcov instrument");
+            raise Xcov_Exit_Exc;
+         end if;
          if Verbose then
             Run_Diagnostics (Self.TU);
          end if;