RTC: fix play crash or no stream for rtmp2rtc tips.(#2728)

[chundonglinlin]

If I don't publish stream, when I play RTC stream and SRS will crash. And if I publish RTC stream, when I play RTC stream and SRS don't distribute RTC stream.Config as follows:

vhost __defaultVhost__ {
    rtc {
        enabled     on;
        # @see https://github.com/ossrs/srs/wiki/v4_CN_WebRTC#rtmp-to-rtc
        rtmp_to_rtc off;
        # @see https://github.com/ossrs/srs/wiki/v4_CN_WebRTC#rtc-to-rtmp
        rtc_to_rtmp off;
    }
}

[codecov-commenter]

Codecov Report

Merging #2863 (f5b601d) into 4.0release (08910e5) will not change coverage.
The diff coverage is 0.00%.

@@             Coverage Diff             @@
##           4.0release    #2863   +/-   ##
===========================================
  Coverage       60.22%   60.22%           
===========================================
  Files             121      121           
  Lines           51070    51070           
===========================================
  Hits            30756    30756           
  Misses          20314    20314           

| Impacted Files | Coverage Δ | |'

Translated to English while maintaining the markdown structure:

'| Impacted Files | Coverage Δ | |
|---|---|---|
| trunk/src/app/srs_app_rtc_api.cpp | 0.00% <0.00%> (ø) | |

Translated to English while maintaining the markdown structure:

| trunk/src/app/srs_app_rtc_api.cpp | 0.00% <0.00%> (ø) | |

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| Δ = absolute <relative> (impact), ø = not affected, ? = missing data |

Translated to English while maintaining the markdown structure:

| Δ = absolute <relative> (impact), ø = not affected, ? = missing data |

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