libCuba: guard memcpy in SobolIni against negative length

GCC's -Wstringop-overflow flagged the memcpy in SobolIni() with a bound
of (size_t)(-4): on the (in practice unreachable) path where the Sobol
generator polynomial 'powers' is 0, the bit-count loop leaves inibits at
its initial -1, so inibits*sizeof underflows. The generator-polynomial
table always has a non-zero first column, so this never happens at run
time, but the compiler cannot prove it.

Guard the copy with 'if (inibits > 0)', which silences the false-positive
warning and hardens the edge case without changing behaviour for valid
input.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/external/libCuba/src/common/Random.c

@@ -102,7 +102,8 @@ static inline void SobolIni(This *t)
     int inibits = -1, bit;
     for( j = powers; j; j >>= 1 ) ++inibits;
 
-    memcpy(pv, pini, inibits*sizeof *pini);
+    if( inibits > 0 )
+      memcpy(pv, pini, inibits*sizeof *pini);
     pini += 8;
 
     for( bit = inibits; bit <= nbits; ++bit ) {