Cellular: Fix for AT handler consume to tag

If sequence from buffer contains tag but symbol before tag is same as
first symbol of the tag, then the tag wasn't detected.

For example, "\r\n" tag was not found from "\r\r\nOK" sequence.

Author: Mirela Chirica

UNITTESTS/features/cellular/framework/AT/athandler/athandlertest.cpp

@@ -1052,6 +1052,27 @@ TEST_F(TestATHandler, test_ATHandler_consume_to_stop_tag)
 
     ATHandler at(&fh1, que, 0, ",");
     EXPECT_TRUE(at.consume_to_stop_tag());
+
+    at.clear_error();
+    char table1[] = "\r\n\r\r\r\nOOK\r\n";
+    at.flush();
+    filehandle_stub_table = table1;
+    filehandle_stub_table_pos = 0;
+    mbed_poll_stub::revents_value = POLLIN;
+    mbed_poll_stub::int_value = 1;
+    char buf1[6];
+    at.resp_start();
+    EXPECT_TRUE(at.consume_to_stop_tag());
+
+    at.clear_error();
+    char table2[] = "OKOK\r\n";
+    at.flush();
+    filehandle_stub_table = table2;
+    filehandle_stub_table_pos = 0;
+    mbed_poll_stub::revents_value = POLLIN;
+    mbed_poll_stub::int_value = 1;
+    char buf2[6];
+    EXPECT_TRUE(at.consume_to_stop_tag());
 }
 
 TEST_F(TestATHandler, test_ATHandler_set_debug)

features/cellular/framework/AT/ATHandler.cpp

@@ -926,16 +926,16 @@ bool ATHandler::consume_to_tag(const char *tag, bool consume_tag)
         int c = get_char();
         if (c == -1) {
             break;
-        } else if (c == tag[match_pos]) {
+        // compares c against tag at current position and if this match fails
+        // compares c against tag[0] and also resets match_pos to 0
+        } else if (c == tag[match_pos] || ((match_pos = 1) && (c == tag[--match_pos]))) {
             match_pos++;
             if (match_pos == strlen(tag)) {
                 if (!consume_tag) {
                     _recv_pos -= strlen(tag);
                 }
                 return true;
             }
-        } else if (match_pos) {
-            match_pos = 0;
         }
     }
     tr_debug("consume_to_tag not found");