Add check for too complex patterns.

```
-- bug since 2.5 (C-stack overflow)
do
  local function f (size)
    local s = string.rep("a", size)
    local p = string.rep(".?", size)
    return pcall(string.match, s, p)
  end
  local r, m = f(80)
  assert(r and #m == 80)
  r, m = f(200000)
  assert(not r and string.find(m, "too complex"), tostring(r)..", "..tostring(m))
end
```

src/core/org/luaj/vm2/lib/StringLib.java

@@ -803,6 +803,8 @@ public class StringLib extends TwoArgFunction {
 	private static final LuaString SPECIALS = valueOf("^$*+?.([%-");
 	private static final int MAX_CAPTURES = 32;
 	
+	private static final int MAXCCALLS = 200;
+	
 	private static final int CAP_UNFINISHED = -1;
 	private static final int CAP_POSITION = -2;
 	
@@ -846,6 +848,7 @@ public class StringLib extends TwoArgFunction {
 	};
 	
 	static class MatchState {
+		int matchdepth;  /* control for recursive depth (to avoid C stack overflow) */
 		final LuaString s;
 		final LuaString p;
 		final Varargs args;
@@ -860,10 +863,12 @@ public class StringLib extends TwoArgFunction {
 			this.level = 0;
 			this.cinit = new int[ MAX_CAPTURES ];
 			this.clen = new int[ MAX_CAPTURES ];
+			this.matchdepth = MAXCCALLS;
 		}
 		
 		void reset() {
 			level = 0;
+			this.matchdepth = MAXCCALLS;
 		}
 		
 		private void add_s( Buffer lbuf, LuaString news, int soff, int e ) {
@@ -1052,6 +1057,8 @@ public class StringLib extends TwoArgFunction {
 		 * where match ends, otherwise returns -1.
 		 */
 		int match( int soffset, int poffset ) {
+			if (matchdepth-- == 0) error("pattern too complex");
+			try {
 				while ( true ) {
 					// Check if we are at the end of the pattern -
 					// equivalent to the '\0' case in the C version, but our pattern
@@ -1128,6 +1135,9 @@ public class StringLib extends TwoArgFunction {
 						continue;
 					}
 				}
+			} finally {
+				matchdepth++;
+			}
 		}
 		
 		int max_expand( int soff, int poff, int ep ) {