Fixed issue: tonumber("-") returns 0 #6

core/src/main/java/org/luaj/vm2/LuaString.java

@@ -794,6 +794,11 @@ public class LuaString extends LuaValue {
 	private double scanlong( int base, int start, int end ) {
 		long x = 0;
 		boolean neg = (m_bytes[start] == '-');
+		// --- fix starts here ---
+		if(neg && m_bytes.length == 1) {
+			return Double.NaN; // this is only a '-' sign, no parsing any number is required
+		}
+		// --- fix ends here ---
 		for ( int i=(neg?start+1:start); i<end; i++ ) {
 			int digit = m_bytes[i] - (base<=10||(m_bytes[i]>='0'&&m_bytes[i]<='9')? '0':
 					m_bytes[i]>='A'&&m_bytes[i]<='Z'? ('A'-10): ('a'-10));