fix bug where line was empty in staq compiler

Signed-off-by: Alex McCaskey <mccaskeyaj@ornl.gov>

quantum/plugins/staq/compiler/staq_compiler.cpp

@@ -275,9 +275,10 @@ std::shared_ptr<IR> StaqCompiler::compile(const std::string &src,
 
 
   std::string preamble = "include \"qelib1.inc\";";
-  auto preamble_start = src.find(preamble);
+  auto preamble_start = _src.find(preamble);
 
- // Add any required missing pre-defines that we 
+
+  // Add any required missing pre-defines that we 
   // know the impl for.
   std::vector<std::string> added;
   std::string extra_insts = "\n";
@@ -288,7 +289,8 @@ std::shared_ptr<IR> StaqCompiler::compile(const std::string &src,
         line.find("include") == std::string::npos &&
         line.find("measure") == std::string::npos &&
         line.find("qreg") == std::string::npos &&
-        line.find("creg") == std::string::npos) {
+        line.find("creg") == std::string::npos
+        && !line.empty()) {
       auto inst_name = split(line, ' ')[0];
       if (inst_name.find("(") != std::string::npos) {
         inst_name = inst_name.substr(0, inst_name.find("("));