Fix out of bounds read in slarrv

This was originally reported as JuliaLang/julia#42415.
I've tracked this down to an our of bounds read on the following line:

https://github.com/Reference-LAPACK/lapack/blob/44ecb6a5ff821b1cbb39f8cc2166cb098e060b4d/SRC/slarrv.f#L423

In the crashing example, `M` is `0`, causing `slarrv` to read uninitialized
memory from the work array. I believe the `0` for `M` is correct and indeed,
the documentation above supports that `M` may be zero:

https://github.com/Reference-LAPACK/lapack/blob/44ecb6a5ff821b1cbb39f8cc2166cb098e060b4d/SRC/slarrv.f#L113-L116

I believe it may be sufficient to early-out this function as suggested
in this PR. However, I have limited context for the full routine here,
so I would appreciate a sanity check.

SRC/slarrv.f

@@ -395,6 +395,13 @@ SUBROUTINE SLARRV( N, VL, VU, D, L, PIVMIN,
       CALL SLASET( 'Full', N, ZUSEDW, ZERO, ZERO,
      $                    Z(1,ZUSEDL), LDZ )
 
+*
+*     Early exit if we have no eigenvalues
+*
+      IF( M.LE.0 ) THEN
+         RETURN
+      END IF
+
       EPS = SLAMCH( 'Precision' )
       RQTOL = TWO * EPS
 *