Prevent segfaults on unterminated strings

A single quote input was causing segfaults due to sneaking past the
string termination logic. This patch corrects that lapse in conditional
by only parsing strings where a closing quote was found. All other
strings are rejected as invalid.

Big thanks to Jean-Charles Campagne (@jccampagne) for reporting the
issue.

c_src/decoder.c

@@ -246,11 +246,12 @@ dec_string(Decoder* d, ERL_NIF_TERM* value)
         }
     }
 
-parse:
-    if(d->p[d->i-1] != '\"') {
-        return 0;
-    }
+    // The goto above ensures that we only
+    // hit this when a string is not terminated
+    // correctly.
+    return 0;
 
+parse:
     if(!has_escape) {
         *value = enif_make_sub_binary(d->env, d->arg, st, (d->i - st - 1));
         return 1;

test/004-strings.t

@@ -6,7 +6,7 @@ main([]) ->
     code:add_pathz("ebin"),
     code:add_pathz("test"),
 
-    etap:plan(118),
+    etap:plan(119),
     util:test_good(good()),
     util:test_good(uescaped(), [uescape]),
     util:test_errors(errors()),
@@ -50,6 +50,7 @@ uescaped() ->
 
 errors() ->
     [
+        "\"",
         <<"\"foo">>,
         <<"\"", 0, "\"">>,
         <<"\"\\g\"">>,