net/http: delete unnecessary string concatenation
net/http: delete unnecessary string concatenation

In the existing implementation, if pattern is an empty string,
program calls a panic with the message which is a concatenation of
"http: invalid pattern " and pattern.
In this case, pattern is an empty, so the commit removes
this concatenation and the trailing space.

Fixes: #21102

Change-Id: I49f58b52d835311a6ac642de871eb15646e48a54
Reviewed-on: https://go-review.googlesource.com/50350
Reviewed-by: Dmitri Shuralyov <0c2a7749d909c8353e59c3f9c6a4d19a5ce000c1@gmail.com>
Reviewed-by: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>
Run-TryBot: Tom Bergan <3b77d7da34779f829901a664ac7c87d255b041af@google.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

codestation/go,cloudflare/go,polyverse-security/go,nawawi/go,josharian/go,josharian/go,golang/go,Cofyc/go,josharian/go,polyverse-security/go,CAFxX/go,CAFxX/go,akutz/go,akutz/go,nawawi/go,codestation/go,cloudflare/go,akutz/go,polyverse-security/go,codestation/go,cloudflare/go,CAFxX/go,akutz/go,nawawi/go,josharian/go,CAFxX/go,cloudflare/go,akutz/go,nawawi/go,akutz/go,Cofyc/go,codestation/go,akutz/go,CAFxX/go,Cofyc/go,CAFxX/go,josharian/go,josharian/go,cloudflare/go,cloudflare/go,polyverse-security/go,Cofyc/go,CAFxX/go,polyverse-security/go,nawawi/go,golang/go,josharian/go,golang/go,Cofyc/go,polyverse-security/go,golang/go,Cofyc/go,codestation/go,golang/go,codestation/go,cloudflare/go,Cofyc/go,CAFxX/go,golang/go,christopher-henderson/Go,nawawi/go,codestation/go,cloudflare/go,Cofyc/go,akutz/go,josharian/go,golang/go,nawawi/go,golang/go,codestation/go,polyverse-security/go,nawawi/go,polyverse-security/go