py/compile: Give the compiler a hint about num nodes being non-zero.

Without this, newer versions of gcc (eg 11.2.0) used with -O2 can warn
about `q_ptr` being maybe uninitialized, because it doesn't know that there
is at least one qstr being written in to this (alloca'd) memory.

As part of this, change the type of `n` to `size_t` so the compiler knows
it's unsigned and can generate better code.

Code size change for this commit:

       bare-arm:   -28 -0.049%
    minimal x86:    -4 -0.002%
       unix x64:    +0 +0.000%
    unix nanbox:   -16 -0.003%
          stm32:   -24 -0.006% PYBV10
         cc3200:   -32 -0.017%
        esp8266:    +8 +0.001% GENERIC
          esp32:   -52 -0.003% GENERIC
            nrf:   -24 -0.013% pca10040
            rp2:   -32 -0.006% PICO
           samd:   -28 -0.020% ADAFRUIT_ITSYBITSY_M4_EXPRESS

Signed-off-by: Damien George <damien@micropython.org>

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