Avoid Oracle errors for OCI_DESCRIBE_ONLY for non-SELECTs

Don't do anything in prepare_for_describe() if the statement is not a
SELECT, as it is not necessary and results in "ORA-24333: zero iteration
count" error, since there's nothing to describe.

This commit is best viewed ignoring whitespace-only changes.

Closes SOCI#1119.

src/backends/oracle/statement.cpp

@@ -163,21 +163,33 @@ std::string oracle_statement_backend::rewrite_for_procedure_call(
 
 int oracle_statement_backend::prepare_for_describe()
 {
-    sword res = OCIStmtExecute(session_.svchp_, stmtp_, session_.errhp_,
-        1, 0, 0, 0, OCI_DESCRIBE_ONLY);
+    ub2 statementType = OCI_STMT_UNKNOWN;
+    sword res = OCIAttrGet(static_cast<dvoid*>(stmtp_),
+        static_cast<ub4>(OCI_HTYPE_STMT), static_cast<dvoid*>(&statementType),
+        0, static_cast<ub4>(OCI_ATTR_STMT_TYPE), session_.errhp_);
     if (res != OCI_SUCCESS)
     {
         throw_oracle_soci_error(res, session_.errhp_);
     }
 
-    int cols;
-    res = OCIAttrGet(static_cast<dvoid*>(stmtp_),
-        static_cast<ub4>(OCI_HTYPE_STMT), static_cast<dvoid*>(&cols),
-        0, static_cast<ub4>(OCI_ATTR_PARAM_COUNT), session_.errhp_);
-
-    if (res != OCI_SUCCESS)
+    int cols = 0;
+    if (statementType == OCI_STMT_SELECT)
     {
-        throw_oracle_soci_error(res, session_.errhp_);
+        res = OCIStmtExecute(session_.svchp_, stmtp_, session_.errhp_,
+            1, 0, 0, 0, OCI_DESCRIBE_ONLY);
+        if (res != OCI_SUCCESS)
+        {
+            throw_oracle_soci_error(res, session_.errhp_);
+        }
+
+        res = OCIAttrGet(static_cast<dvoid*>(stmtp_),
+            static_cast<ub4>(OCI_HTYPE_STMT), static_cast<dvoid*>(&cols),
+            0, static_cast<ub4>(OCI_ATTR_PARAM_COUNT), session_.errhp_);
+
+        if (res != OCI_SUCCESS)
+        {
+            throw_oracle_soci_error(res, session_.errhp_);
+        }
     }
 
     return cols;