Trac #8450: work around unplottable points in plot_vector_field().

Now that many of our plotting functions are using fast_callable()
WITHOUT domain=float to support complex numbers that appear in
intermediate computations, we need to work around some plot points
that used to be NaN (as a result of domain=float) but now throw errors
as we try to compute them.

One such instance is in plot_vector_field(), where the lines

  xvec_array.append(f(x, y))

and

  yvec_array.append(g(x, y))

can fail to evaluate either f or g. This commit adds two try/except
blocks to catch ValueError (calling math.sqrt on a negative number)
and ZeroDivisionError (function undefined at some point), and turn
them into NaN. The hope is that this leaves the behavior unchanged,
since domain=float would have forced these evaluations to return NaN
in the past.

src/sage/plot/plot_field.py

@@ -268,8 +268,16 @@ def plot_vector_field(f_g, xrange, yrange, **options):
         for y in xsrange(*ranges[1], include_endpoint=True):
             xpos_array.append(x)
             ypos_array.append(y)
-            xvec_array.append(f(x, y))
-            yvec_array.append(g(x, y))
+            try:
+                fxy = f(x, y)
+            except (ValueError, ZeroDivisionError):
+                fxy = float("nan")
+            xvec_array.append(fxy)
+            try:
+                gxy = g(x, y)
+            except (ValueError, ZeroDivisionError):
+                gxy = float("nan")
+            yvec_array.append(gxy)
 
     import numpy
     xvec_array = numpy.ma.masked_invalid(numpy.array(xvec_array, dtype=float))