r300/compiler: Silence uninitialized variable warning.

r300/compiler: Silence uninitialized variable warning.

The variable loops would be used uninitialized if it ever processed a
RC_OPCODE_ENDLOOP case first.

This patch initalizes the loops variable to NULL and adds an assert at
the RC_OPCODE_ENDLOOP case that loops isn't NULL.

Silence the following GCC warning.
r3xx_vertprog.c: In function 'translate_vertex_program':
r3xx_vertprog.c:469: warning: 'loops' may be used uninitialized in this function