fix some undefined behavior

[Korshikov]

I'm a member of the Pinguem.ru competition on finding errors in open source projects. A bug, found using PVS-Studio.
Errors:
compton/src/c2.c 600 err V774 The 'tstr' pointer was used after the memory was released.
Warning:
compton/src/common.h 1835 warn V575 The potential null pointer is passed into 'memcpy' function. Inspect the first argument.
compton/src/common.h 1836 warn V575 The potential null pointer is passed into 'memcpy' function. Inspect the first argument.
compton/src/common.h 1837 warn V575 The potential null pointer is passed into 'memcpy' function. Inspect the first argument.
compton/src/compton.c 7139 warn V575 The potential null pointer is passed into 'memcpy' function. Inspect the first argument.

[Korshikov]

One more error:
compton/src/opengl.c 1665 err V674 The '0.2' literal of the 'double' type is subtracted from a variable of the 'int' type. Consider inspecting the 'z -= 0.2' expression.
compton/src/opengl.c 1694 err V674 The '0.1' literal of the 'double' type is subtracted from a variable of the 'int' type. Consider inspecting the 'z -= 0.1' expression.

but I not understand what must be done in idea. So I can't fix it without understanding.