internal/gocommand: skip ill-formed env vars in cmdDebugStr

Fixes golang/vscode-go#2260

Change-Id: I4dbdb1d582e3b434bce9f0e923899d8af6a43f3d
Reviewed-on: https://go-review.googlesource.com/c/tools/+/408376
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>

internal/gocommand/invoke.go

@@ -264,8 +264,10 @@ func cmdDebugStr(cmd *exec.Cmd) string {
 	env := make(map[string]string)
 	for _, kv := range cmd.Env {
 		split := strings.SplitN(kv, "=", 2)
-		k, v := split[0], split[1]
-		env[k] = v
+		if len(split) == 2 {
+			k, v := split[0], split[1]
+			env[k] = v
+		}
 	}
 
 	var args []string