Fix panic=abort when compiling with plugins

[alexcrichton]

Closes #2738

[rust-highfive]

@alexcrichton: no appropriate reviewer found, use r? to override

[alexcrichton]

r? @brson

[brson]

@bors r+

[bors]

📌 Commit e9c6b0a has been approved by brson

[bors]

⌛ Testing commit e9c6b0a with merge d402bc7...

[bors]

💔 Test failed - cargo-linux-32

[alexcrichton]

@bors: retry

On Fri, Aug 5, 2016 at 10:17 AM, bors notifications@github.com wrote:

💔 Test failed - cargo-linux-32
https://buildbot.rust-lang.org/builders/cargo-linux-32/builds/1768

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[bors]

⌛ Testing commit e9c6b0a with merge c7bb4f3...

[bors]

💔 Test failed - cargo-win-gnu-32

[alexcrichton]

@bors: retry

On Saturday, August 6, 2016, bors notifications@github.com wrote:

💔 Test failed - cargo-win-gnu-32
https://buildbot.rust-lang.org/builders/cargo-win-gnu-32/builds/647

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[bors]

⌛ Testing commit e9c6b0a with merge c917e94...

[bors]

☀️ Test successful - cargo-cross-linux, cargo-linux-32, cargo-linux-64, cargo-mac-32, cargo-mac-64, cargo-win-gnu-32, cargo-win-gnu-64, cargo-win-msvc-32, cargo-win-msvc-64
Approved by: brson
Pushing c917e94 to master...