tests: Fix test-suite failure on Alpha

tests: Fix test-suite failure on Alpha

Pulseaudio fails to build on the Alpha architecture due to a failure
in the volume-test of the test suite.  I had reported this to the
Debian bug tracker [1] but the maintainer has asked that I forward the
patch to this mail list.  The failure in volume-test occurs because it
is compiled with -ffast-math which implies -ffinite-math-only of which
the gcc manual states that it optimizes for floating-point arithmetic
with the assumption that arguments and results are not NaNs or
+/-infinity, and futher notes that it may result in incorrect output.
On the Alpha platform that is somewhat an understatement as the use of
non-finite floating-point arithmetic with -ffinite-math-only results in
a floating-point exception and the termination of the program.

The volume-test converts volumes into decibels (so a zero volume
becomes a negative infinity) and proceeds to add two volumes (in
decibels), thus does arithmetic with non-finite floating point numbers
despite being compiled with -ffast-math!

I attach a patch that protects against the arithmetic with non-finite
numbers for your consideration.  With that patch the test-suite passes
on Alpha.

Cheers
Michael.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=798248