C library/getenv test: actually try to dereference the result

Our regression test would previously succeed even when no implementation
of `getenv` was provided.

Author: tautschnig

regression/cbmc-library/getenv-01/main.c

@@ -5,4 +5,8 @@ int main()
   char *something;
   // there should not be any overflow in there
   something = getenv("HOME");
+  if(something != NULL)
+    return something[0];
+  else
+    return 0;
 }

regression/cbmc-library/getenv-01/test.desc

@@ -1,6 +1,6 @@
 CORE
 main.c
---signed-overflow-check --unsigned-overflow-check --pointer-overflow-check --pointer-check --bounds-check
+--signed-overflow-check --unsigned-overflow-check --pointer-overflow-check --pointer-check --bounds-check --memory-leak-check
 ^EXIT=0$
 ^SIGNAL=0$
 ^VERIFICATION SUCCESSFUL$