Support Windows UNC paths with fat jars

Update `Launcher` and `JarFile` to correctly deal with Windows UNC
paths.

Fixes gh-1641

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/Launcher.java

@@ -142,7 +142,7 @@ public abstract class Launcher {
 		ProtectionDomain protectionDomain = getClass().getProtectionDomain();
 		CodeSource codeSource = protectionDomain.getCodeSource();
 		URI location = (codeSource == null ? null : codeSource.getLocation().toURI());
-		String path = (location == null ? null : location.getPath());
+		String path = (location == null ? null : location.getSchemeSpecificPart());
 		if (path == null) {
 			throw new IllegalStateException("Unable to determine code source archive");
 		}

spring-boot-tools/spring-boot-loader/src/main/java/org/springframework/boot/loader/jar/JarFile.java

@@ -424,6 +424,7 @@ public class JarFile extends java.util.jar.JarFile implements Iterable<JarEntryD
 		if (this.url == null) {
 			Handler handler = new Handler(this);
 			String file = this.rootFile.getFile().toURI() + this.pathFromRoot + "!/";
+			file = file.replace("file:////", "file://"); // Fix UNC paths
 			this.url = new URL("jar", "", -1, file, handler);
 		}
 		return this.url;