gdb/testsuite: Don't attempt tests if they fail to compile

In the gdb.base/whatis-ptype-typedefs.exp test, if the test program
fails to compile, don't run the tests.

gdb/testsuite/ChangeLog:

	* gdb.base/whatis-ptype-typedefs.exp: Don't run tests if we failed
	to prepare.
	(prepare): Return 0 on error, 1 on success.

gdb/testsuite/ChangeLog

@@ -1,3 +1,9 @@
+2018-01-12  Andrew Burgess  <andrew.burgess@embecosm.com>
+
+	* gdb.base/whatis-ptype-typedefs.exp: Don't run tests if we failed
+	to prepare.
+	(prepare): Return 0 on error, 1 on success.
+
 2018-01-12  Pedro Alves  <palves@redhat.com>
 
 	* gdb.base/continue-after-aborted-step-over.c: New.

gdb/testsuite/gdb.base/whatis-ptype-typedefs.exp

@@ -45,13 +45,15 @@ proc prepare {lang} {
 
     if { [prepare_for_testing "failed to prepare" \
 	      ${out} [list $srcfile] $options] } {
-	return -1
+	return 0
     }
 
     if ![runto_main] then {
 	fail "can't run to main"
 	return 0
     }
+
+    return 1
 }
 
 # The following list is layed out as a table.  It is composed by
@@ -300,6 +302,7 @@ proc run_tests {lang} {
 }
 
 foreach_with_prefix lang {"c" "c++"} {
-    prepare $lang
+    if { [prepare $lang] } then {
 	run_tests $lang
+    }
 }