Fix JSON parser error found by fuzz

fuzz/json_fuzzer_seed_corpus/f297bc5680b3730d3386caa8dee36bf5a853516b

@@ -0,0 +1 @@
+"\,,,,,p,,-\,,,,

fuzz/qtest/fuzz.test

@@ -16,7 +16,7 @@ my @fuzzers = (
     ['dct' => 1],
     ['flate' => 1],
     ['hex' => 1],
-    ['json' => 37],
+    ['json' => 38],
     ['lzw' => 2],
     ['pngpredictor' => 1],
     ['runlength' => 6],

libqpdf/JSON.cc

@@ -1246,7 +1246,8 @@ JSONParser::handleToken()
         break;
 
     default:
-        throw std::logic_error("JSONParser::handleToken : non-terminal lexer state encountered");
+        throw std::runtime_error(
+            "JSON: offset " + std::to_string(offset) + ": premature end of input");
         break;
     }