gsm_7bit_decode: Handle the case of the last charachter being an escape

If i + 1 == length and it is an escape charachter we would try
to read rtext[length] which is one after the field we have allocated.
diff --git a/src/gsm_utils.c b/src/gsm_utils.c
index fb69377..3a378ac 100644
--- a/src/gsm_utils.c
+++ b/src/gsm_utils.c
@@ -65,7 +65,7 @@
 	}
 	for(i = 0; i < length; i++){
 		/* this is an extension character */
-		if(rtext[i] == 0x1b){
+		if(rtext[i] == 0x1b && i + 1 < length){
 			tmp = rtext[i+1];
 			*(text++) = gsm_7bit_alphabet[0x7f + tmp];
 			i++;