rdp_plugin.c: Fix dereferencing of NULL variable when profile name is empty

Description

The conditional was backwards so the while loop tried to dereference p when it was NULL instead of when it was not NULL.

This also fixes the thread name being empty instead of RemmRDP:<NONAM> when no name is specified.

Related Issue

Fixes: #2559 (closed)

Motivation and Context

Fixes segfault

How Has This Been Tested?

Confirmed that the thread name is correctly set when the profile name is empty:

(gdb) info locals
rfi = 0x555555c30070
remminafile = 0x555555817610
profile_name = <optimized out>
p = 0x0
thname = "RemmRDP:<NONAM>"
c = <optimized out>
nthname = 15
(gdb)

❯ ps -eTl | grep RemmRDP
1 t  1000  280278  280289  280271  0  80   0 - 141575 -     pts/7    00:00:00 RemmRDP:<NONAM>

and when the profile name is set to Test:

(gdb) info locals
rfi = 0x5555563b4400
remminafile = 0x5555565a8190
profile_name = <optimized out>
p = 0x55555650cab4 ""
thname = "RemmRDP:Test\000\177\000"
c = <optimized out>
nthname = 12

❯ ps -eTl | grep RemmRDP
1 t  1000  280278  280592  280271  0  80   0 - 286284 -     pts/7    00:00:00 RemmRDP:Test

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.