mgcp: Address scanf issue already found by cppcheck and now ASAN

Use the right size for scanf.

=================================================================
==6106== ERROR: AddressSanitizer: unknown-crash on address 0xbffff4b0 at pc 0xb69d87fd bp 0xbffff248 sp 0xbffff21c
WRITE of size 65 at 0xbffff4b0 thread T0
    #0 0xb69d87fc (/usr/lib/i386-linux-gnu/libasan.so.0.0.0+0xa7fc)
    #1 0xb69d9239 (/usr/lib/i386-linux-gnu/libasan.so.0.0.0+0xb239)
    #2 0xb69d92d6 (/usr/lib/i386-linux-gnu/libasan.so.0.0.0+0xb2d6)
    #3 0x804f151 (/home/ich/source/gsm/openbsc/openbsc/tests/mgcp/mgcp_test+0x804f151)
    #4 0x80531e8 (/home/ich/source/gsm/openbsc/openbsc/tests/mgcp/mgcp_test+0x80531e8)
    #5 0x8051e6f (/home/ich/source/gsm/openbsc/openbsc/tests/mgcp/mgcp_test+0x8051e6f)
    #6 0x8049b0a (/home/ich/source/gsm/openbsc/openbsc/tests/mgcp/mgcp_test+0x8049b0a)
    #7 0x804bd9e (/home/ich/source/gsm/openbsc/openbsc/tests/mgcp/mgcp_test+0x804bd9e)
    #8 0xb6778a62 (/lib/i386-linux-gnu/i686/cmov/libc-2.18.so+0x19a62)
    #9 0x8049330 (/home/ich/source/gsm/openbsc/openbsc/tests/mgcp/mgcp_test+0x8049330)
Address 0xbffff4b0 is located at offset 416 in frame <parse_sdp_data> of T0's stack:
  This frame has 8 object(s):
    [32, 36) 'audio_payload'
    [96, 100) 'payload'
    [160, 164) 'channels'
    [224, 228) 'ptime'
    [288, 292) 'port'
    [352, 368) 'ipv4'
    [416, 480) 'audio_name'
    [512, 576) 'audio_codec'
HINT: this may be a false positive if your program uses some custom stack unwind mechanism or swapcontext
      (longjmp and C++ exceptions *are*
diff --git a/openbsc/src/libmgcp/mgcp_protocol.c b/openbsc/src/libmgcp/mgcp_protocol.c
index d4088a1..1a88a84 100644
--- a/openbsc/src/libmgcp/mgcp_protocol.c
+++ b/openbsc/src/libmgcp/mgcp_protocol.c
@@ -621,7 +621,7 @@
 			if (audio_payload == -1)
 				break;
 
-			if (sscanf(line, "a=rtpmap:%d %64s",
+			if (sscanf(line, "a=rtpmap:%d %63s",
 				   &payload, audio_name) == 2) {
 				if (payload != audio_payload)
 					break;