Fix crash in Ada "catch exception"

PR ada/24919 concerns a crash that Tom de Vries noticed when running
the "catch_ex" Ada test case.  He sent a test executable and tracked
this down to commit f21c2bd7b7 ("Fix Fortran regression with variables
in nested functions").

Looking at that patch, you can see the obvious error:

    -  return 0;
    +  return true;

Oops!  This patch fixes the bug.

Tested on x86-64 Fedora 29.

gdb/ChangeLog
2019-09-20  Tom Tromey  <tromey@adacore.com>

	PR ada/24919:
	* block.c (contained_in): Fix final return value.

gdb/block.c

@@ -86,7 +86,7 @@ contained_in (const struct block *a, const struct block *b,
     }
   while (a != NULL);
 
-  return true;
+  return false;
 }