tools: libxl: Simplify logic in libxl__realloc

tools: libxl: Simplify logic in libxl__realloc

Replace the loop exit and separate test for loop overrun with an
assert in the loop body.

This simplifies the code.  It also (hopefully) avoids Coverity
thinking that gc->alloc_maxsize might change, resulting in the loop
failing to find the right answer but also failing to abort.

(gc->alloc_maxsize can't change because gcs are all singlethreaded:
either they are on the stack of a specific thread, or they belong to
an ao and are covered by the ctx lock.)

Signed-off-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Acked-by: Ian Campbell <ian.campbell@citrix.com>