Fix potential overflow bug.

Author: SeanRBurton

src/dfa.rs

@@ -66,9 +66,12 @@ use sparse::SparseSet;
 pub fn can_exec(insts: &Program) -> bool {
     use prog::Inst::*;
     // If for some reason we manage to allocate a regex program with more
-    // than u32::MAX instructions, then we can't execute the DFA because we
-    // use 32 bit instruction pointers for memory savings.
-    if insts.len() > ::std::u32::MAX as usize {
+    // than i32::MAX instructions, then we can't execute the DFA because we
+    // use 32 bit instruction pointer deltas for memory savings.
+    // If i32::MAX is the largest positive delta,
+    // then -i32::MAX == i32::MIN + 1 is the largest negative delta,
+    // and we are OK to use 32 bits.
+    if insts.len() > ::std::i32::MAX as usize {
         return false;
     }
     for inst in insts {