Fix bugs identified by running clang's `scan-build-16` on the MLVWM source

[morgant]

I recently learned about Clang's scan-build tool for identifying bugs and ran the following on my OpenBSD amd64/7.5-stable workstation:

scan-build-16 -o tmp/scan-build make -j4

Which identified 29 bugs, including:

• (x6) Dereference of null pointer
• (x1) Garbage return value
• (x1) Result of operation is garbage or undefined
• (x1) Uninitialized argument value
• (x1) Unix API
• (x1) Memory leak
• (x2) Allocator sizeof operand mismatch
• (x14) Dead assignment
• (x2) Dead nested assignment

mlvwm - scan-build results.pdf

[morgant]

I have addressed all the "dead assignments" identified in borders.c, menus.c, and event.c with the exception of baseHeight & baseWidth in event.c's ConstrainSize() as they do appear to be used. For the those, I'm wondering if it's a parsing issue since the formatting is quite wonky and there's a possibility that there are shift-jis characters in there.

[morgant]

I'm also skipping the "dead nested assignments" identified in event.c for similar reasons.

[morgant]

I have fixed a memory leak in misc.c's ReadIcon() due to a new Icon being allocated before checking to see if the icon file at the path actually exists.

[morgant]

I have fixed a NULL value being passed to strcmp() in menus.c's ChangeMenuLabel().

[morgant]

I have fixed a possible uninitialized argument value in misc.c's ReadIcon().

[morgant]

I have fixed all the potential dereference of null pointer values in menu.c's press_menu() & ChoiseMenu().