Fix segfault when invalid field given in reporting commands.

Fix segfault when invalid field given in reporting commands.

Problem is dm_report_init() may return NULL and subsequent call to
dm_report_set_output_field_name_prefix() doesn't handle NULL value.

Example:
	pvs --nameprefixes --rows --unquoted --noheadings -opv_name,fred
  Logical Volume Fields
  ---------------------
    lv_uuid              - Unique identifier
    lv_name              - Name.  LVs created for internal use are enclosed in brackets.
 ...

  Physical Volume Segment Fields
  ------------------------------
    pvseg_start          - Physical Extent number of start of segment.
    pvseg_size           - Number of extents in segment.

  Unrecognised field: fred
Segmentation fault