Fix crash bugs on connect and disconnect

[alrvid]

Code that would return early for the control endpoint had been removed in the
ST port by Arm, which led to a null pointer being used as the this pointer in a
USBDeviceConnected object, which caused a write to a small offset from
address 0, which caused a fault. Fixing this triggered a new bug in another
location, but the second commit in this PR fixes that one too, by adding an extra
guard clause that checks for nullptr.

[github-actions]

Memory usage change @ dba59a0

Board	flash	%	RAM for global variables	%
arduino:mbed_giga:giga	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_opta:opta	0 - 0	0.0 - 0.0	0 - 0	0.0 - 0.0
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A

Click for full report table

Board	examples/DirList flash	%	examples/DirList RAM for global variables	%	examples/FileRead flash	%	examples/FileRead RAM for global variables	%	examples/FileWrite flash	%	examples/FileWrite RAM for global variables	%	examples/OptaDirList flash	%	examples/OptaDirList RAM for global variables	%	examples/PortentaOTA flash	%	examples/PortentaOTA RAM for global variables	%
arduino:mbed_giga:giga	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:mbed_opta:opta	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0	0	0.0
arduino:mbed_portenta:envie_m7	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A	N/A

Click for full report CSV

Board,examples/DirList<br>flash,%,examples/DirList<br>RAM for global variables,%,examples/FileRead<br>flash,%,examples/FileRead<br>RAM for global variables,%,examples/FileWrite<br>flash,%,examples/FileWrite<br>RAM for global variables,%,examples/OptaDirList<br>flash,%,examples/OptaDirList<br>RAM for global variables,%,examples/PortentaOTA<br>flash,%,examples/PortentaOTA<br>RAM for global variables,%
arduino:mbed_giga:giga,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:mbed_opta:opta,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0,0,0.0
arduino:mbed_portenta:envie_m7,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A,N/A

[sebromero]

LGTM