Deal correctly with short strings

The parseMacAddress function anticipates only properly formed
MAC addresses (6 hexadecimal octets separated by ":").  This
change properly deals with situations where the string is
shorter than expected, making sure that the passed in char*
reference in parseHexByte never exceeds the end of the string.

BUG: 28164077
TEST: Added a main function:

int main(int argc, char **argv) {
   unsigned char addr[6];
   if (argc > 1) {
      memset(addr, 0, sizeof(addr));
      parseMacAddress(argv[1], addr);
      printf("Result: %02x:%02x:%02x:%02x:%02x:%02x\n",
              addr[0], addr[1], addr[2], addr[3], addr[4], addr[5]);
   }
}

Tested with "", "a" "ab" "ab:c" "abxc".

Change-Id: I0db8d0037e48b62333d475296a45b22ab0efe386
(cherry picked from commit 1921acbf2c82dd0813b3734f2609fe6f971c9c2f)
diff --git a/service/jni/com_android_server_wifi_WifiNative.cpp b/service/jni/com_android_server_wifi_WifiNative.cpp
index 31166e3..acc02d6 100644
--- a/service/jni/com_android_server_wifi_WifiNative.cpp
+++ b/service/jni/com_android_server_wifi_WifiNative.cpp
@@ -697,15 +697,23 @@
 }
 
 static byte parseHexByte(const char * &str) {
+    if (str[0] == '\0') {
+        ALOGE("Passed an empty string");
+        return 0;
+    }
     byte b = parseHexChar(str[0]);
-    if (str[1] == ':' || str[1] == '\0') {
-        str += 2;
-        return b;
+    if (str[1] == '\0' || str[1] == ':') {
+        str ++;
     } else {
         b = b << 4 | parseHexChar(str[1]);
-        str += 3;
-        return b;
+        str += 2;
     }
+
+    // Skip trailing delimiter if not at the end of the string.
+    if (str[0] != '\0') {
+        str++;
+    }
+    return b;
 }
 
 static void parseMacAddress(const char *str, mac_addr addr) {