glsl: free or reuse memory allocated for TF varying

When a shader program is de-serialized the gl_shader_program passed in
may actually still hold memory allocations for the transform feedback
varyings. If that is the case, free the varying names and reallocate
the new storage for the names array.

This fixes a memory leak:
Direct leak of 48 byte(s) in 6 object(s) allocated from:
 in malloc (/usr/lib64/gcc/x86_64-pc-linux-gnu/7.3.0/libasan.so+0xdb880)
 in transform_feedback_varyings ../../samba/mesa/src/mesa/main/transformfeedback.c:875
 in _mesa_TransformFeedbackVaryings ../../samba/mesa/src/mesa/main/transformfeedback.c:985
 ...
Indirect leak of 42 byte(s) in 6 object(s) allocated from:
  in __interceptor_strdup (/usr/lib64/gcc/x86_64-pc-linux-gnu/7.3.0/libasan.so+0x761c8)
  in transform_feedback_varyings ../../samba/mesa/src/mesa/main/transformfeedback.c:887
  in _mesa_TransformFeedbackVaryings ../../samba/mesa/src/mesa/main/transformfeedback.c:985

Fixes: ab2643e4
   glsl: serialize data from glTransformFeedbackVaryings

Signed-off-by: Gert Wollny <gert.wollny@collabora.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>
(cherry picked from commit f5d05370)