AST-2018-003: Crash with an invalid SDP fmtp attribute

pjproject's fmtp retrieval function failed to catch invalid fmtp attributes.
Because of this Asterisk would crash if given an SDP with an invalid fmtp
attribute.

When retrieving the format this patch now makes sure the fmtp attribute is
available. If not available it now returns an error status.

ASTERISK-27583 #close

Change-Id: I5cebe000ce2d846cae3af33b6d72c416e51caf2f

third-party/pjproject/patches/0071-sdp_fmtp_attr.patch

@@ -0,0 +1,34 @@
+diff --git a/pjmedia/src/pjmedia/sdp.c b/pjmedia/src/pjmedia/sdp.c
+index a3dd80b..6117e07 100644
+--- a/pjmedia/src/pjmedia/sdp.c
++++ b/pjmedia/src/pjmedia/sdp.c
+@@ -256,7 +256,8 @@ PJ_DEF(pj_status_t) pjmedia_sdp_attr_get_rtpmap( const pjmedia_sdp_attr *attr,
+ 
+     PJ_ASSERT_RETURN(pj_strcmp2(&attr->name, "rtpmap")==0, PJ_EINVALIDOP);
+ 
+-    PJ_ASSERT_RETURN(attr->value.slen != 0, PJMEDIA_SDP_EINATTR);
++    if (attr->value.slen == 0)
++        return PJMEDIA_SDP_EINATTR;
+ 
+     init_sdp_parser();
+ 
+@@ -341,6 +342,9 @@ PJ_DEF(pj_status_t) pjmedia_sdp_attr_get_fmtp( const pjmedia_sdp_attr *attr,
+ 
+     PJ_ASSERT_RETURN(pj_strcmp2(&attr->name, "fmtp")==0, PJ_EINVALIDOP);
+ 
++    if (attr->value.slen == 0)
++        return PJMEDIA_SDP_EINATTR;
++
+     /* fmtp BNF:
+      *	a=fmtp:<format> <format specific parameter>
+      */
+@@ -379,6 +383,9 @@ PJ_DEF(pj_status_t) pjmedia_sdp_attr_get_rtcp(const pjmedia_sdp_attr *attr,
+ 
+     PJ_ASSERT_RETURN(pj_strcmp2(&attr->name, "rtcp")==0, PJ_EINVALIDOP);
+ 
++    if (attr->value.slen == 0)
++        return PJMEDIA_SDP_EINATTR;
++
+     init_sdp_parser();
+ 
+     /* fmtp BNF: